How to early tailgate locks compare to late tailgate locks?

What makes them not interchangeable?

 

when i replaced the black late bay tail gate lock on mine for a chrome early one i had to unscrew the arm on the back of the lock and turn it 90 degrees then refit

 

nothing as far as im aware i had chrome metal tailgate lock on my 79my bay (one of the last) from a pre74 van fitted staight on if i remember