As is with no provenance it sure does not help,,,
But what happens when it does turn out to be something special.
It can be removed without evidence of its removal, Most of these "MR-3DB" grip frames were oversized and full of porosity.
…a little history…
Fitting them became labor intensive with contamination to the cylinder frame and having effect on the bluing process,,,
the reason a lot were sold as an accessory w/ a wide trigger.
NICE EARLY Old Army you have there, these guns
early BRASS GRIP FRAME'd Old Army's are really neat, IMO. Get a letter and hope it comes back as a BGF Gun, will increase its value but One has to part with it to recognize the gain, and that usually comes with a big-ol dose of regret…
And even if it doesn't come back as leaving the factory as a BFG, it's still a early gun that has a period correct BFG accessory.
FWIW; MR-3DB is in the casting, not stamped, yours is 100% correct. Sometimes the MR-3DB can be very faint and difficult to see, especially amongst all the "rough stuff".