I just put a tang sight on a new Winchester 1873. The sight I used was a new Lyman. The box said it was for a 94 but it fits the 73 perfectly. I used 2 front mounting screws: one that came with the Lyman and one that I think came from a Marbles sight. Two front screws
for a tang sight for Winchester will do it. All Winchester lever guns used the same front mounting screw. It is 3/16 -36. Both holes on a 73 are front mounting size.
I shot mine this weekend right before the flood arrived. The sight worked great. A vintage Lyman for a 94 wouldn't work because of the larger rear screw hole. I think it is a nice touch that Winchester put 2 holes for the tang sight like an original 73 and not one plus the tang screw like a 94. I'm sure Buffalo Arms or Marbles can send the right screws. I just happened to have something that worked.
Jthbjr
In very wet Art, Texas