I have acquired a Model 75 Sporting Rifle from the late 40s in very fine condition which has the Lyman 57 ES rear sight installed.  However, it is missing any aperture disc which should thread into the orifice. 

Are the aperture discs for this model sight interchangeable with other Lyman sights or should I be looking for particular ones?  If so, are they marked so as to differentiate them from other discs?  Are these still obtainable?

Any response is welcome.

Ed