Thanks alot for the advice guys, I did suspect that the head may have been reused or swapped out from another bike...the last owner has proven to be pretty reckless/ignorant in this regard. But thats good news really, as I can get a replacement '16 head and reuse the cylinders+pistons without too much worry, although it sounds as if MC18 cylinders might work out if I can find a pair...
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'87 MC16