I know I'm nitpicking here, but "Apple did the same thing between the 2011/12 15” Unibody MacBook Pro" is not correct. The 15" versions of 2011 and 2012 are completely different, because 2011 is the old A1286 and 2012 is the first retina A1398.
There is of course the 13" A1278 '11 and '12 that are very similar but unsure if these boards or the same.
Maybe you meant A1398 Mid 2012 and early 2013? These have interchangeable boards, just like the '16/'17 series.
Also sorry to resurrect an old thread ;)