Originally Posted by Alk8944

Goes back a long, long way. My recollection is the original designation of Remington primers was 1, 2, 9, etc, when they were the old mercuric (fulminate of mercury) black powder compound. This changed sometime in the 1920s-1930s when the change was made to the non-mercuric, non-corrosive "Kleanbore" priming compound which is lead styphnate based. The 1/2 was added to distinguish between the two types.

This does not apply to the 5 1/2, as there never was a mercuric size 5 (SP Magnum) primer, but it provides consistancy in the numbering system. Apparently Remington expects reloaders to consult reference materials, like loading manuals, to understand the application of their various primers. Similarly, they added an M, making the 9 1/2M Large Rifle Magnum primer. You are supposed to know, from references, that it is the Large Rifle Magnum primer.

Every loading manual I can think of has a reference section, and lists of various manufacturers primers are identified as to type.