Pete E. wrote:

So on my new-to-me S-C 250...I tried a pair of the 3M315 Gates poly-flex belts with their external cogs.

Fit was fine, but it made some odd noise as though the cogs were rubbing along the sides of the pulley grooves.

Kept them on for 2-3 hours with the machine running but the noise did not go away.

Went back to o-rings which are very quiet on this machine.

When I get a chance, I will try the cogged Gates belts on my S-C Electria 120 to see if I can use them there.  Will report results, then.
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I've had similar results. The cogged belts fit on some Electra 120's and not others in which they are too tight to get on. Also, the "O" rings are quiet and more tolerant off fit variances.  Also, slightly different size "O" pair of rings can be played with to get good alignments.
It helps that the "O" rings are far cheaper.
j