It's presumed to be at least one of the (two) factually known NR Masers, one of which possibly wasn't needed or used in NR which is the one that became the ANH Hero..and many other films/shows before it, haha. There's still no proven evidence that there were more than those two made with the same modifications. Although they claimed (3, or 4 now?) that were modified in the same manner, yet no proof of such claims. All of us here and the general public even have that we can factually state, is there were two Mausers proven to be made in that manner by Bapty, no photos or proof otherwise.
Until better, higher res images surface of the 'Bunker/Bizarro Blaster', we can't really know more than what we can see sadly. All we can see is that it's clearly a bull-barreled Mauser (only the Bapty NR/onward Mausers are currently known to have that), and the mount and HW scope was visibly present too, though reversed and mounted to an ROTJ style mount as noticed by
OdiWan72 I think?
The Mauser has obviously been molded at least twice, first known is the Merr Sonn with the NR lower, the other for the ESB/ROTJ stunt blasters with the Hero ANH upper and lower. No proof or evidence as of yet suggesting that the mount and/or scope were ever molded and cast and used at all.
But the length of the Bunker blaster's barrel suggests the likelier possibility that it's not a stunt casting being used and modified with that ridiculous barrel and such. The real Mausers had threads in the bull-barrel to accommodate for the extension. Though, all of the known stunt casts from that Hero ANH Mauser were molded with the FH on it. Thus making the bunker blaster less likely to be a casting, since removing the section where the FH is cast in would shorten the visible Bizarro blaster's bull-barrel substantially, and noticeably.
I have absolutely no clue as to why they made that crazy blaster that way, it removes full continuity completely from Han's blaster throughout the film.
Maybe the scene was intended to show a blaster stolen from an imperial officer or something? But who knows.
Interesting when the scope and mount come in to play, they've only been seen in two films so far, ST and SW (but, possibly/likely on the bunker blaster in ROTJ too).
We have no solid evidence (or any implication even) of any known castings from the full ANH Hero at all in the first film, none.
Only the Merr Sonn with the different lower. But our unknowing doesn't mean they didn't exist, it's possible that we just simply haven't seen them or didn't make it into the film(s). But as far as screen-used goes, all of the current evidence and reference seems to show that the ANH Hero appears to be the same odd, unique, swapped lower, lumped, Pat's patched Mauser throughout the entirety of ANH. The same Mauser that all of us here have been studying with extreme passion over all of the...well, decades now..
-Carson