I'm thinking all the L frame guns have the logo on the left, beneath the release latch. On the K and N frame guns, depends on the model, but many about the same time the pinned barrels were dropped. For instance, the M66-1 (P&R), has the logo on the side plate on the right, but the M66-2 has the logo on the left. That would make the change around 1982. Just a SWAG on my part, based on observations.