b) different followers--- early/straight magazines (plastic- right hump) and later/curved
magazines (chromed- left hump); early magazine springs on early/straight magazines,
and later magazine springs on later/curved magazines; later magazine springs on
early/straight magazines need modification of loops at top
Another member had the same question a while back, but I could not find his post in the Advanced Search--- maybe he will comment.
And the springs are physically different:
Older straight mags have round spring; current curved mags have oval springs.
The Followers are shaped accordingly to accept the corresponding springs, so they aren’t interchangeable.