Chris,
Hmmmm. I expected to see a pile of responses on this Q by now. Since no one else wants to take a stab at it, I'll offer my two bit's worth.
That disc ends up captive between the flywheel and the crank and, as far as I can tell, serves to guide the input shaft snout into the pilot bushing on the Hydra-Matic equipped engines. Once installed, nothing touches it (or at least, it had [i]better[/i] not). Since the torus side of the flywheel is submerged in ATF while running and likely [i]all[/i] of the time (realizing that it partially drains into the pan during non-run periods) it doesn't seem logical that it would be there to retain lubricant, hence my hypothesis that it's just a guide/snout-support during assembly.
That's my hypothesis and I'm stickin' to it.
Frank

I believe that is a splash shield to retain some transmission fluid on the pilot bushing. Should be sealed in place with pliobond. See Autotrans.com for details

It's official name is Retainer-crankshaft bushing, identifying its purpose. I would not reinstall transmission without this!

David is correct. The puzzle is that there are two different part number retainers used for the Hydramatics. early 51-52 and late 52-54. The bushing is the same for all Hydramatics..Why a different retainer? Not to mention "why a retainer at all"? Perhaps they were worried that the bushing might stay attached to the transmission and get lost before the transmission is replaced.? Of course, don't forget the flywheel to crank must be sealed with a suitable sealer.
hmmmm. ???

I plan on putting it back in but wonder if gluing it in is necessary as it could have been an assembly line procedure to temporarily keep it in place.