This is mounted just rear of the side-plate on Model 1899 sn346874 that in all other respects appears to be a legit carbine, has a 1901 Carbine rear sight.

There are 2 threaded bolts into the receiver (does not use side-plate screw). Before I remove it & patch the holes would like to know what it is. I have 20 Krags & and maybe a dozen Lyman/Redfield/Rice/Williams peep sights but have never seen this.

Whatever it is it's probably incomplete, missing something, or is the latch/holder for another piece that is missing. Lever moves back & forth as though it would latch/release something.

Any ideas? Thanks all!

Got it, thanks very much! I have a Lyman 57K in my box of stuff (not mounted) that requires drill/tap but mounts on the right side like a lot of others that require drill/tap. This one is new to me.

I did see the slight mod to the stock but was not aware the side-plate needed modification too.

May try to find the parts to a 33/34K on e-bay or something before removing it altogether & patching the holes.

Thanks again!