Split Bus Electric Panel Question
The origin of Split-Bus panels

So, think 1920.  Lighting is the "killer app" that is making everyone electrify their homes.  Plug-in appliances are still coming along, but by and large 30A is plenty for most people just doing lighting and the few plug-in appliances of the day, with 60A for those with a big electric appliance. 

Now, think 1950.  Postwar, they're building houses like crazy, and they have new-fangled "all electric" appliances - electric ranges, electric dryers, electric water heaters - some houses even have electric heat!  And why not? Nuclear fission plants are in the pipeline, and just around the corner is nuclear fusion -- clean, safe and too cheap to meter. The old 30A service (for mainly lights) or 60A service (for lights and maybe one big electric appliance) just doesn't cut it anymore. 

The problem is, service equipment hasn't been keeping pace. Miniature circuit breakers exist, but the design mentality is still stuck in 1920 - with 60A being the largest main breaker that's really made in any production.  And these new homes need at least 100A, and 100A main breakers are nosebleed-expensive industrial tier products. 

"Well", say builders.  "How about if we split the main breaker function to several several affordable main breakers?  You have to throw them all to de-energize the house.  Figuring on electric range, dryer, water heater, A/C, and a fifth to feed an internal 'subpanel' for traditional 'lighting' and recep loads". And the panel builders said "It needs to be an even number."  And NFPA said "alright, six it is.  You must be able to turn the power off with 6 hand movements".  Which also happens to be a rule in industrial power. 

And so they built in the panel with a "built-in subpanel" for "lighting" (read: normal 120/240V loads). 

So the top 12 spaces, the "Rule of Six" area, are for major (electrical) appliances that pull 240V.  Thus the 6 need 12 spaces.  You can put single-pole or even double-stuff breakers in there, but you won't be able to use all 12 spaces if you do. 

Is the panel legal as pictured?

I count 1-2-3-4-5 throws.  Yes, you can do that and put 1 more breaker in the Rule of Six area.  However then you are done.  You may find that stifling, so I recommend putting small loads in the subpanel area. 

It also remains that Rule of Six/split-bus panels are scary dangerous, because if  you have 50A range + 30A AC + 30A water + 30A dryer + 60A for the lighting section + a 100A breaker to the Tesla charger, you may notice that's 300A of "main breakers" on a 125A service.  As houses power more stuff, you have a higher chance of blowing right past the safety limits on the panel - and nothing catches this on a split-bus panel! 

Meanwhile 100A and larger breakers have come down massively in price.   So I like to see Rule of Six panels converted into "Rule of One" panels one of several ways: 


Given that exterior disconnects are Code requirements in 2020, fit an exterior main breaker and put the Rule of Six panel downstream of it.  Now it's just another panel, and you can continue it in service forever. (and your CH panel is excellent and you have every reason to do so).  
Fit a single breaker into the Rule of Six area, and have that power a new subpanel.  Now you have a single breaker shutoff.