The older steel receiver BLRs (pre-'81), with the protruding magazine, have a concave shaped receiver - the receiver is thinner in the middle than it is at the ends where it meets the stock. This thinner receiver makes it easier to carry when one is holding the rifle with one hand around the receiver, IMO. From 1981 on, the receivers were made straight-sided - the extra width permitted the redesign of the magazines to a wider width, which subsequently shortened them to the point where they are more or less flush with the bottom of the receiver.
The newer, aluminum receivers introduce in 1995 maintained the straight-sided receiver. The 1995 models also introduced the folding hammer as a safety feature, which, unfortunately, prohibits the use of a hammmer extension that I find very useful on my older BLRs.
1977 .358 BLR
1980 .358 BLR - original owner
1987 .243 '81 BLR
1991 7mm-08 '81 BLR