BB spool mounts

Can you explain what these even are? No 73 A body came with a factory B/RB did it?

I'm sorry I assumed that as soon as a person read factory/stock they would know right off that they weren't for an A-body cause they wasn't orginal big block in a spool mount K-frame...and know that they are for B and C bodies.

That being said I've seen some folks with pictures using these with and without mods....how they were able to do it & not be able to do it, I do not know.