"inconsequential dimensions"? The problem with being lost is that we can be deep into it before we realize.
Rock of Ages and/or Collins can (could have?), no doubt, make (made) any special dimension surface plate that a customer would pay for. However, if you look at any listing of common SP sizes, 3" thick 24 x 36 is not a common thickness. Too thin to resist self weight, let alone live load of work piece(s) and metrology equipment. 4" thick is only common in import SPs. I would expect a 24 x 36 to be 6" thick. Based on the OP, I had suspicion that what you have is actually a 4 ledge surface plate. The concept of "ledges" is to allow easy clamping of work/fixtures to the SP. The ledges, at ≈3" thick, are stiff enough to support any reasonable load due to ≈3" overhang. The fact that your unit has labels from two manufacturers may be contrary evidence to my "4 ledge" theory", but not necessarily. The plate may have been manufactured by one and modified/reworked/calibrated by the other. The presence of some type of "epoxy" material isn't conclusive to me that you have two discreet plates bonded into an assembly.
In addition to whatever I might glean from seeing your piece in person, I would look for manufacturers serial numbers and call the to request info regarding what was built. I don't know if that info is retained, but I would call to find out.
If you have two discreet plates, I don't know what the perpetrator had in mind. The bottom of SPs are not commonly precision flats so now you would have a rough surface sitting on a precision surface? Reason? I can't imagine. The idea of bonding two thin plates together to create a stiffer unit would be news to me (but a good question to ask the manufacturers). Is that a valid manufacturing technique?
I'm guessing that your plate came from an industrial environment. Although bonding two discrete plates together is easy to understand in a hobby context, I bet it's very rare, or nonexistent (possible factory manufacturing technique not withstanding) in an industrial user environment related to precision metrology (maybe they just needed some ballast).
If you intend to end up with a precision surface plate, familiarize yourself with 'airy points' and implement a three point (small area elastomeric pads) mounting system. I would ask a manufacturer if the airy points are the same for a 4 ledge plate as they are for a no ledge plate (I don't think so).
Good luck.
On edit: I just saw numerous photos that were not posted (I missed?) when I read earlier. That is a 4 ledge plate. It isn't a bonded assembly. If you look at the "Rock of Ages" tag, it says RESURFACED. Conclusion; it's a Collins plate that was resurfaced/upgraded/calibrated by Rock of Ages. It's a nice piece. Make/buy a great stand for it (I see at least one elastomeric pad bonded at an airy point) and enjoy.