Part number 353247-S-A2 is shown in the 1969 Mustang Electrical Assembly Manual (AM0028) page 10 (ref E9-125-3) item 2. I've found no other reference to that part number. For the same application in 1970 (AM0033 pg 13, ref E0-125.3), grommet part number 379160-S was used. It's measurements (from 69 Ford Standard and Utility Parts, AM0205, pg 58) are 1/4ID,1/2OD,5/16T(NO OTHER DIMENSIONS). The service replacement for both 69 and 70 applications is 379009-S (same page as 379160-S).
It is common for Ford Engineering to find a part to use, only to discover that the source cannot supply it for any number of reasons, so a replacement is used. The next model year gets the same treatment, service somehow solves all those quirks with something else. Many times the replacement will "fit", same size mounting hole, same size hole for the choke cable, but the thickness is different. Those are observations, not documented. That's the best information I can find.
Jim