Probably this

National 414045, sold by many vendors including Amazon. This may be what is leaking if you are talking about leaking oil near brake plate area. It will fit 1953 3100.

The Procedure for 1954 is here. (may be same for 1953)
The other link from Julian is for up to 1951.

Here is an exploded view that was found on Stovebolt forum. Scroll down to last post by Jorb and click on the link. Seal 5.822. That is a the parts catalog number which is just for reference. It would get you to the part number, but all those old part numbers have been changed many times, so use the National Part number or equivalent.

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