Many people try and mislead people point to reference books as having all the answers. Many times the writers of these books lift information from someone else and continue spreading bad information. This link is one:
M1 Carbine Hammer B R in A Double Box for Rock Ola Unissued Part | eBay
I harp on this particular hammer because, you can not always say a W is a Winchester, R is a Rockola and U is not always an Underwood.