Nothing inherently wrong with release year naming scheme, but the same generation might get bumps over a few years so you still want the 'generation' classification.
cMP, nMP, mMP and then if you wanted to cMP09 or mMP18 and mMP20.
Apple's own internal designation like 4.1, 5.1 and 6.1 sees quite a bit of use, but most people would probably figure out the actual model quicker if it said nMP13, as long as they had a basic understanding of what the abbreviation stood for in the first place.
Apart from that the obvious choice seems to be MacP0rn.