Remington Model 121 Fieldmaster Screw Size

[Aeromarine]

I recently acquired a well used Model 121. Unfortunately, at some point over its life someone added a left side of the receiver bolt on scope mount. Ick! In addition they removed both the front sight and the rear leaf open sight (held on by two screws). I have found it impossible to locate the original missing parts anywhere and finally ordered after market replacement sights from Numrich Arms. However, I have not yet been able to find the two screws that hold on the rear sight. Does anyone know what thread size and screw length Remington used? Any advice would be gratefully appreciated! Thanks!!