It is extremely unlikely that an 1892 vintage Model 1873 was factory fitted for a sound suppressor (silencer). Maxim was one of the first (if not the first) commercial makers of silencers, and they did not begin making them until circa 1907.
If you can post pictures of your rifle with the cap installed and removed, we might be able to provide more information.
WACA 6571L, Historian & Board of Director Member
November 7, 1987
May 24, 2012