I asked this question before and this is the answer I received then. "The NM cylinders don't work in OM pistols. Not so much the length, which can be fitted (i.e. sanded to the slightly shorter length), but that the ratchet wheel (term??) is a different size (smaller on NM). OM = .625", NM = .58" IIRC." It seems like someone also showed a picture of the ratchet that appeared to verify the above statement.
Those dimensions are correct; in fact my ratchet diameters vary from .540" all the way to .623". That's on 4 NMs and 5 OMs (actually 13 different cylinders because I measured both cylinders on the 4 NM convertibles. And the small diameter ratchet cylinders will work in any single six once fitted for length however are a little more likely to need the pawl fitted.
The good news is all of the 5 OM larger ratchet cylinders will fit in the 4 NM guns based on measurements (some were too long to actually be placed in the cylinder windows). That's because at least all 4 of my NM guns have a ratchet cut of at least .640".
So the only issue of fitting non-original cylinders is the 2 lengths (OA and cyl + ratchet), not the NM vs. OM issue. However, in my experience, guns closer to the vintage of the gun tend to be closer fits or 'drop ins'. And as Dan so aply illustrated.