Definitely an oversight, but for the topic of the brakes it really doesn't matter. You can't use any BBP brakes with any SBP axles, regardless of which rear axle it is. It does make a difference for the axles of course, because if I'm not mistaken the SBP 7.25" rear axle can't swap axles with the later BBP 7.25 rear axle for other reasons. That's just going off of what I've read though, I've never had a SBP 7.25" in any of my cars. All of my A-bodies either had SBP 8 3/4's or BBP 7.25's, and I got rid of the BBP 7.25 pretty quickly.
Nope!
You can not interchange SBP and BBP backing plates or drums. SBP backing plates must be used with SBP axles and drums. If you re-drill the SBP axles to the 5x4.5" pattern, you must re-drill the SBP drums as well. You cannot use stock BBP drums (10x2.5") with SBP backing plates or re-drilled SBP axles.
It's a matched set. If you want to use the 10x2.5" BBP drums, you must use the 10x2.5" backing plates and BBP axles (not SBP axles redrilled to BBP!!!). If you want to use 10x1.75" SBP drums, you must use 10x1.75" backing plates and SBP axles. If you re-drill a set of SBP axles to 5x4.5", or buy a set of aftermarket axles with the SBP axle flange offset, you must still use the 10x1.75" backing plates and drums and re-drill the drums for 5x4.5.
The BBP axles have a different flange offset, as shown in the picture I posted above. That means the backing plates also have a different offset, because the drums are a different distance from the axle housing where the backing plates mount. The difference in the axle flange offset is not the same as the difference between the width of the BBP and SBP shoes either, so, there's no combination that works other than keeping the SBP brakes paired with SBP axles and the BBP brakes paired with BBP axles.