Assuming it is a 3 inch diameter seal as both my 54 and 61 were here are a couple numbers to try that I found on my trailers.
Chicago Rawhide 18808
The best fit I found was a seal from Trailer hitch, hitches and bike rack (800)298-8924
which I got to by referencing other numbers. It is 3.005 diameter. I don't have their part number here but the E trailer number does cross ref. back to kelsey hayes We are travelling right now and I don't have all my notes. At least one of the other numbers is 2.998 diameter and might work loose if no adhesive is put on it.
A google search for your part number should bring up numerous cross reference guides as well as some airforums threads
I also found that E trailer was the only source for drive in dust caps to fit my 61 which are 2.25 diameter
UPDATE: This is the etrailer seal
If your seal is not 3 inch diameter just ignore most of what I wrote