72 vs 73 front shock tube differences

[Tricky D]

I have a 72 that someone started as a project and never finished.

It looks like I have Boge struts, but possibly with Bilstein inserts.

I've been looking around and note that some front shocks go up to 72, and some are 73 and later.

What is the difference between the 72 and 73 front shock tubes? What changes would be required to retrofit a 73 and up housing into a 72?

Thanks.