I have never seen a 73-up seat that looked like it would fit in a 67-72, but I have not seen them all.
I have a seat in my garage a former co-worker gave me, he said it was out of a 75, and looks exactly like a 67-72 type, but he is not a reliable person. I think it likely came out of a 71 or 72.
The 73-up seats I recollect will take a lot of modifying to get bolted in...I could be wrong.
Clear as mud?
Anyone?