Essentially, as it was explained to us at H*nda, there were the 'pre-epa' carbs and then the 'epa' carbs, with 2 different idle circuit methodologies.
The older/original 'pre-epa' carbs premixed bleed air at the pilot jet, which then went to the 'idle' screw. That screw controlled total idle fuel flow.
The later 'epa' setup changed that, so that the screw now controlled how much bleed air went to the pilot 'percolator' or 'aerator' (part of pilot jet with lateral holes), hence was now called the 'mix' screw.
These new 'mix' screws had rotation-restriction caps on them, to maintain compliance.
It never dawned on me that the other brands were going thru similar nightmares, now with 5twins docs/diagrams I can see Yamaha had similar design changes. The H*nda changes were primarily in the carb body, bowl swap was often not a problem. The Yamaha BS carbs, with the critical jet/mix zone in the bowl, have to have proper body/bowl match. Then, as 5twins shows, a different slide arrangement appears in this timeframe. I believe GggGary's spot on with his 'recurring issue' prediction.