In a parts stash I recently received from a hepful, retiring soul who use to run Y blocks, this adapter plate was among the goodies. TD-51-11 IMG_0502 by danman55 posted Apr 9, 2019 at 11:20 AM IMG_0503 by danman55 posted Apr 9, 2019 at 11:20 AM IMG_0505 by danman55 posted Apr 9, 2019 at 11:20 AM IMG_0506 by danman55 posted Apr 9, 2019 at 11:20 AM Maybe if someone has a Transdapt catalog from tha 60's or 70's you may know what it is for? Thanks for the help. The center hole is stepped down from 4.75 to 4.5 inches.
without looking it up, I would guess that it's the ford to ford adapter, mates early to late (the change happened around 1965). See how it look like the universal transmission?
also the -11 matches the limited info I have, meaning the adapter fits the 65 and later ford car transmissions https://www.jalopyjournal.com/forum/threads/tech-transdapt-adapter-numbers.78455/