I ended up here through a garage sale purchase of a super solid (this thing is a tank) unbadged 2x12 cabinet with a few G12T-75s in it and some MOD 50s. I was told it was a custom build. It was pretty cheap, and I bought it mainly to flip the speakers and fund an extension cab for my Pathfinder.
Eventually, I noticed that there was a shadow and holes left by the badge in what would have been the upper left corner when the cab was vertical. These seemed to match the Sunn nameplate on some cabs I saw that had sold on eBay and Reverb. Thanks to this forum, I was able to identify the dimensions as matching a 1969-70 piggyback Solarus cab. Most other details line up as well (tolex, corners, grillcloth, handle), so my best guess is that this is what it is. The casters were moved to the side at some point, and it also looks like the jack cup may have been replaced (along with all the trim bolts for that side).
Pics are below. If there's something that either definitively marks or doesn't mark it as a Sunn cab please let me know. If it is one, I'll probably restore it to vertical try to get a nameplate and some decent speakers (maybe even try for a Solarus head if I'm feeling ambitious). I figure whether I keep it or sell it, it probably deserves its name back, so if anyone has suggestions on finding a nameplate (I think it should be the one with the "R" mark, yes?), please let me know.
Thanks for helping out the new guy!