zermatt7: The pre 1981 BLRs have a concaved-shaped receiver - if you look at your 1975 model, the receiver is narrower in the middle than it is at both ends where it mates with the stocks. Thus, the magazine is narrower and deeper, as Hairtrigger explained. The '81 BLRs have a straight-sided receiver that permitted a wider and, thusly, shallower magazine that is basically flush with the bottom of the receiver. Consequently, the magazines are not interchangeable between the two models.
Enjoy that BLR!
pagris
1977 .358 BLR
1980 .358 BLR - original owner
1987 .243 '81 BLR
1991 7mm-08 '81 BLR