Distributor vacuum diaphragm assembly [E1TZ-12370-A] - NOS

https://www.ebay.com/itm/NOS-FORD-MOTORCRAFT-DD-871-E1TZ-12370-A-1981-TRUCKDISTRIBUTOR-DIAPHRAGM-ASSY/283774315516?hash=item4212446bfc:g:pioAAOSwZTNePH4U

That's a really hard thing to tell what it goes on.  The issue is that the MPC lists parts of a distributor in a table like below, and you can see from the highlighting that the vacuum diaphragm in question fits two different distributor numbers in this snippet alone.  But that table is 12 pages long and there are several distributors that use that diaphragm.

Once you have the distributor's part number you take that #, not the diaphragm's, and search the MPC for that.  You'll get hits in one or more calibration parts list(s), and that will tell you what vehicles use that diaphragm.

Clear as mud?