Seems to me that every time I tried to get a pinion seal for my 50 ford sedan, I ended up with a mercury pinion seal. You might try some of the parts stores and see if that is still the case. Sorry, I don't know the part number though. Maybe this will help.
My National book says, ****er 41. 49-54. 5778. Redi-sleeve 99155 10 Bolt. 49-54. 6371. Hope that helps.