Thanks to Mark Hammons of Toploader Heaven for the first two images!
FE engines require a spacer plate between the rear of the engine block and the transmission bellhousing. The picture below shows the plate used for both automatic and manual transmission applications starting with the 1968 model year. Notice the three small holes at top, the large hole at bottom, and the FoMoCo stamping at right. Mark reports that these plates typically do not have any stamped numbers or date codes. The large hole to the left is for the starter. Mark has also noticed that a good number of these plates have a strip of either cork or weatherstripping-like rubber along the bottom edge, perhaps to provide a seal and to help keep rattling noises down. According to Tom Cherry and the 1975 Master Parts Catalog, the service part number for this plate is C3AZ-7007-F.
An older spacer plate that is commonly (but mistakenly) thought to have been used with manual transmissions is shown below. Notice the single large hole at top instead of the three smaller holes found in the -F plate. The stamping around the crankshaft hole is slightly different, and there's no FoMoCo stamping at right. Mark reports that most of the plates that he's seen like this have a stamped date code. Tom Cherry reports that the service part number for this plate is C3AZ-7007-D.
The confusion may exist because Ford did indeed use different spacers depending on the transmission during the 1960s. Tom has noted that the "12/63 parts book shows a C3AZ-7007-B for all applications on an FE and the August 65 revision shows a C3AZ-7007-D for an auto trans and a C3AZ-7007-B for a stick shift. In the 65-67 parts book (1/67) they had refined that to one part and it was the C3AZ-7007-D except for special performance" ... "Starting in 68 they eliminated all the other versions as there was no need for them because the -F was needed to clear the new machining protocol and it still fit all the older engines as well".
Thanks, Tom, for helping to set the record straight!
Tony Pang recently shared a picture of a plate that looks like a hybrid of the two shown above. Note the stamped area around the crankshaft opening (similar to the -D plate) and the three plug holes at top (similar to the -F plate):