The issue I found is the '72 frame rear loop has a larger radius rear bend - I'm guessing to accommodate the wider 'D' section rear mudguard, since earlier frames used a narrower 'C' section. They both measure 255mm at the midway bend, but the '70 curves in more from the mid-point back.
'70 vs '72 frame loop comparison
The effect is the '72 frame fouls the early FB tail moulding - it's not 100% clear in the photo due to the rear tyre, but best endeavours etc...
Which goes back to my clearly contentious question - was there a late model-specific FB tail moulding - I'm guessing the one listed in the '71 & 72 parts book must be wider?
If so, presumably it would be OK on later frames...
