Old Models have both cylinders fluted and both unmarked but if you cannot tell the difference from 10' away you should not be handling a firearm.
The rear face is way different. The LR cylinder has a grove around the chambers to clear the firing pin. The WMR cylinder has recessed chambers and stepped chambers as the WMR head spaces off the the case mouth, the LR off the rim.
They're also different diameters....at least enough that you can't stuff a 22 WMR round into a LR cylinder far enough for the cylinder to rotate.
Thanks for the question, gave me an excuse to get the Old (girl) Model out for some handling.
LR pictured on the left, WMR on the right.