I did my calculations and get the same result (~5.5mm). After cutting the collars fit perfectly.
I suppose that I've got collars are for a bigger Quick, but anyhow - longer collars are better then shorter as one can always cut them to necessary length (they could mention that in the manual, though)

You asked about fitting OS.50 to Quick Learner - well, I did that successfully with two modifications: I had to take off ~2mm of the muffler (see below) because the engine sits a little bit low and muffler gets in contact with the right angle rail, also I had to increase the length of the throttle control arm by ~10mm.



Cheers,
Slavik