37 model hubs

there are 3 different types of 37 model hub shells that i know of  1 is nickel plated i think with just a unthreaded hole which is covered up with a clip that goes around the hub. the next one i think is also nickel plated but has a oiling cap that opens up. the other one is also threaded but it has a rubber piece were the oil goes and i think is chrome plated and is of a different shape on the brake arm side.  i presume the latter hub is the most recent.  any ideas as to what years the changes were made. any help appreciated. thanks in advance. william rudolph