fastback radio??

I am digging through an old parts shed looking for an AM radio for a 71 Plymouth duster and I found two kinds of radios out there, the unit pictured is shorter than the other one and has a slant on the back, the other one is totally box shaped but both have the same faceplate and knobs.

Which one would be correct for a 71 duster and why does this one have a slant on the back, any light shed on different radio types would be appreciated