I don't have one here, but here's what I found with a quick search:
"Ok, I took a look at what I have. Couldn't find any original Ford cup plugs. However, I looked at one steel crank I bought from Carl Holbrook years ago and the cup plugs reads "Dorman 7/8 P14C". I have another steel crank that do not have any cup plugs installed. The cup plugs appear to have an Associated Spring logo stamped on them, along with "PC29 7/8". The box for them reads: Pik-A-Nut, Hunckler Products, Inc. Huntington, Indiana 46750. 7/8 expansion plug regular cup plated steel CXP56 Old No., then in larger font "191 0560". Cost .40 ea.
The Retaining Ring is about .952 dia and .043 thick.
The threaded Plug used on the 427 Steel service crank has a part number of 371485-S, and have a major dia of 1.032. They cost $2.95 ea back in 1985."