O.K., this 1947 Chevrolet Service Bulletin, explains it all. According to this bulletin, Dated December 1947, any 1948 Chevrolet 1 1/2, or 2 ton truck, built after Dec. 1947, would have the new splined hub, & axle flange, & any Chevrolet truck built before Dec. 1947, will have the older bolted flange axles.
http://chevy.tocmp.com/bulletins/47csn1201.htm
