'16 and both '18's have curl in the bottom pipe to put both end cans level.

'16, tiny pipes
'18, cans level, not offset in normal v-twin 250 style
'18, mk2 high level version of above
'21, nothing outstanding
'28, big washers in the headers in a futile attempt to restrain the power of the mighty beast that is a stock carded PGMIV.
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