ShopRag - It took a bit to wade through many of my Chevy Master Parts Books to identify the part that I believe you're referring to as Parts Group 5.866 - Gasket, wheel hub cap. For your '54 4100, that should be GM# 3685199 (screenshot below is from a 1954 Chevy Master Parts Catalog).
That GM# is still in use, on many different year group and models, but doesn't exactly match your pictured gasket, as the current available ones do not have the splined axle notches profile. The picture from this ebay ad for
NOS GM3685199 Gaskets shows a cork gasket without the notches. If you look at the rest of the pictures in that ad, they show outside and inside measurements that correspond to yours.
A check of an
original Chevy 1954 Truck advertising pamphlet shows a picture of the 4000 and up rear axle with the splined shaft and a cut away cap with visible gasket (see another screenshot below). That picture doesn't show a "notched gasket". Makes me wonder if perhaps someone created that gasket to make it easy to pull the axle shafts without disturbing the gasket???
Anyhow, hope you can use this info to order some new gaskets.
