How to identify electrical panel busbar rating
This is a 125A panel
From the labeling on your panel, in particular the note about field upgrading 100A mains to 125A by replacing the main breaker and the meter-to-main-breaker wire jumpers, we can tell that this panel has 125A busbars in it, since UL wouldn't have permitted the manufacturer to provide such an upgrade as an option otherwise.
The fact that it's center-fed may or may not be an issue
If your jurisdiction's adopted the 2017 NEC, you're good to go, since it provides new language for this situation in 705.12(B)(2) point 3(d):

(d) A connection at either end, but not both ends, of a
center-fed panelboard in dwellings shall be permitted where
the sum of 125 percent of the power source(s) output circuit
current and the rating of the overcurrent device protecting the
busbar does not exceed 120 percent of the current rating of
the busbar.

Note that this 120% rule is the same as the rule in subpoint 3(b) that'd be used if your panel was end-fed with the solar breaker at the other end; this means that the 120% math works out all the same, and gives us 150A for the combination of main and solar feed breakers.  With a 125A main breaker, this lets you put a 25A solar breaker in, which limits your system to 6kW.
If you have an AHJ that's not quite with the times, you may run into trouble as the standard load calculations tend to be more restrictive when applied to center-fed configurations such as yours.  In that case, I would talk with the AHJ about a written variance to use the above center-fed calculation method.
Before you go...
The one other issue you might have with this panel is finding breakers for it.  Siemens has discontinued the Murray MP line of breakers, and it appears that the other breaker types it is labeled to take are either GFCI (ITE/Siemens QF1-B, Westinghouse/Eaton QPGF, Bryant/Eaton GFCB) or double-stuff (Bryant/Eaton BRD).
If you don't wish to take a minor gamble on New Old Stock Murray/Siemens MP breakers being available and in good shape, I would use a Siemens QP breaker, based on the subsitution letter Siemens provides, although that's subject to AHJ acceptance of the subsitution.  If you can't get your AHJ to accept Siemens breakers in this panel for some odd reason, you can Eaton CL for the solar breaker as that's UL classified to play nice in Murray (including Arrow-Hart/Crouse-Hinds) panels.
If none of those are an option, your AHJ might be willing to accept an Eaton BR (not BRD or GFCB) breaker in the panel based on it being crosslabeled for BRD and GFCB types, but that's a judgement call considering that UL never tested it for that precise configuration.  (Granted, it's not much different from putting a NOS MP120AFC arc fault breaker in the panel, considering it's far too old to be labeled for type MP-AT2 breakers.)