Looking at the MPC (May, 75 ed) under the seal called out in the 70 Mustang Body Assembly Manual, page 17 (that and a couple of other pages are missing from the 69 Body Manual), C4DB-16A238 - factory, C4DZ-16A238-A - service, it shows the application as "69 390 4V" and "70 200 I-6, 428CJ". However, under full size Ford is a referral to base number 8349 which is another seal group, mostly full size Fords, T-Birds, 70-71 Torino (all engines) and 71-73 Mustangs (all engines). The key here is this seal group, 8349, list the clip 371141-S as being used - only for T-Birds and the Torinos.
That's what Ford service lists, and we know Ford factory is sometimes different. This much can be gleaned from the above - the clip was used for radiator seals. How or why Mustangs got the clip is open. The stapler being broke is a very good reason.
Jim