[unruhjonny]

here is my gut-style reaction to the query of belt lengths;
you should have in essence five belt types/lengths:
1: short male / fixed length = rear
2: short female / adjustable length = rear
3: long male / fixed length = front shoulder
4: retracting male = front lap
5: long female / adjustable length = front lap & shoulder (same units)

I seldom see 1970 deluxe/brushed buckles, so it is possible that the rear seat shorter 'female' buckles may have been substituted with the same lengths as the front units - I say this, because that is what your "set" appears to be;
That top 'female' belt in your second picture is (I believe) the rear seat part - NOT a shoulder belt companion.

Note;
The front 'male' lap belts are side specific.