Old GE Panel, OK to Keep?
This panel might be OK as a subpanel, but it will be rather limited by breaker compatibility issues...
What you have here is a rather...ancient GE panel; in fact, it is ancient enough that it dates back to before the modern &quot;double stuff&quot; breaker type in the GE Q-line (the THQP).  Instead, it calls out the thoroughly obsolete and unobtainable type TR breaker as its &quot;double stuff&quot; breaker type, which means that you are restricted to full-width single-pole breakers.  Furthermore, it is old enough that it dates back to well before current generations of the THQL breaker design (we can tell because it calls out type TQL breakers for the single pole, with type TR tandem breakers), so you may run into issues installing new-production THQL (issue RTxxx or newer) breakers; the only way to tell I know of is to try it and see if the breakers seat down fully on the stab.
This issue is better documented in this DIYChatroom post, with the pertinent parts quoted below for posterity:

GE changed the way they set the breaker's seating height. In my panel, you push the breaker down until the top of the molded slot rests on the stab. TQL breakers, THQL Issue UOxxx (probably MJxxx, NExxx and others too), and also Siemens QP and Eaton BR, can work this way, because they all have the same slot profile.
Recent panels have a ridge of insulating material next to the stab; the bottom of the breaker's case rests on the ridge. The stab is not as tall. It doesn't penetrate the breaker as deep. All the above breakers (except TQL which somehow doesn't quite fit) can be mounted. Also Issue RT-xxx of THQL, which doesn't fit the old bus because it can't accommodate the tall stab. Issue RT-xxx is all you will find in retail stock.

Note that even if current issue THQL breakers do not fit in your panel, you should still be able to use UL classified Eaton type CL breakers in it; however, this limits you to 50A branch circuits overall, and also means you can't get dual-function breakers, two-pole arc or ground fault breakers, or other sorts of exotica for that matter.  These limitations may or may not matter specifically to you, but would weigh against retaining this panel if I were in your shoes, especially considering that it will become surplus to need in any case.
But you do need to evict the aliens if you wish to keep it, though!
If you wish to keep this old soldier of a panel in service, you will need to evict all the alien breakers, though; while Eaton BR and Square D Homeline breakers appear to fit, they are not the correct breaker types for this panel and using them can lead to trouble with the breaker-to-busbar contact system down the road.  As I said before, while new THQL breakers may not fit, used or &quot;new old stock&quot; THQL breakers should still work, and so should Eaton CL breakers that are UL classified for GE panels.
As to how it was used in three phase service (or not used, as it turns out)
As folks may have noticed, the OP's loadcenter is nominally a single-phase, three-wire panelboard, yet it was used in a three-phase, four-wire application.  How? The key in most situations, albeit not the OP's, lies in the rightmost of the panel's three wiring diagrams, where it depicts what is known as a delta breaker being applied in this panel.  These oddball breakers are three-pole breakers, but with two jaws and a pigtail for the line-side connection instead of having three jaws, like a normal three pole breaker.  (The closest thing in appearance to a delta breaker in today's usage is a two-pole switching neutral breaker, but those are different internally as there is no overcurrent trip in the neutral pole.)
In this setup, the two line side jaws connected to the panel busbars, which provided the A and C phases in a 120/240V, 3-phase, 4-wire, high leg delta service, while the pigtail was hooked up to the incoming B phase (high leg), allowing a three-phase load to be powered alongside normal single-phase loads, such as when a three-phase air conditioning compressor or a small amount of three-phase machinery was to be powered from the same service that supplied normal single-phase lighting and power loads.
However, delta breakers had several usage limitations, one of the main ones being that they cannot be correctly used if a two-pole main breaker or main switch is upstream of them.  In particular, what would happen if you tried to use a delta breaker downstream of a two-pole main device is that the busses would stay live with the main shutoff and the delta breaker left on, as the B phase (power/high/wild leg) would backfeed the remaining phases (legs) of the service via the three-phase load.
Fortunately, the fact you had a separate main disconnect in the form of a fused switch kept this from being an issue; in fact, it does not appear that your panel ever had any delta breakers in it.  Instead, the three-phase feeder to the AC disconnect was (likely improperly) tapped at the main safety switch, bypassing this panel entirely and allowing it to serve as a single-phase panel fed by the two lighting legs (A and C phases) of your high-leg delta service.
While it was possible to safely use them in main-lug subpanels fed from three-phase main panels or main shutoffs (such as if the B phase was fed to your panel first instead of bypassing it on its way to the air conditioner), in &quot;rule of six&quot; split-bus main panels, or in a backfed main breaker configuration using a delta breaker as the main breaker and spliced pigtails to distribute the B phase, the misapplication issues encountered in main breaker applications, along with the deprecation of the &quot;rule of six&quot; as applied to split-bus loadcenters, mean that delta breakers are no longer permitted to be used in panelboards atop being long-discontinued items.
Nowadays, the notion of the high leg delta itself is almost obsolete; three-phase combination lighting and power services are run at 208Y/120V, using designed-for-purpose three-phase, four-wire distribution equipment, despite the losses it entails when running resistive loads, as 208Y/120V can be subdivided (for say a multi-unit building, or multiple buildings on a single transformer bank) in a balanced fashion even when individual buildings are being fed with split phase service (120/208V 2 phase 3 wire service is commonly provided in dense areas or for individual tenant spaces/units in high-rises and other dense buildings), vs the intentional imbalance of a high leg delta configuration, and also does not introduce the equipment-damage risks that are present in a high-leg delta service due to the fact the &quot;high leg&quot; runs at 208V phase to ground and thus will damage 120V devices if it is accidentally used to power a single-phase circuit.