Help needed with 1961 Chrysler V8 6.3 Engine oil seal

Hi
I am hoping one of your members can help me. I have had the above engine rebuilt and we have a seal left over from an engine gasket set manufactured by FEL-Pro, part number FS 7891PT, the seal has National Mexico 3103 stamped on the rubber. The dimensions are OD 3/3/16" (80mm) ID 2 1/2" (55mm) Depth 5/8" (15mm). Is it possible that this gasket set is supplied with 2 alternate seals, although the packaging does not list alternatives?
Or has the engine builder left this seal out by mistake (unfortunately the engine was rebuilt approx 2 years ago and no-one can remember). Does anyone have any idea where this seal fits?
I also require a thermostat housing for the engine
Any help gratefully received
Cheers, Malcolm
Author: Malcolm