Does any one know where I can find the definitions for the model 66?

Here's a lot of info in one place.

How to Identify/Date a S&W | Northeastshooters.com Forums

The info you seek is in there.

Quick and dirty version:
66 1971 introduced
-1 1977 Gas ring from yoke to cylinder
-2 1982 Eliminate cylinder counterbore

Pinned barrel elimination not noted but usually not present in -2s (Right guys?)

Transiitionals not rare.

Regards JD

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On the whistle draw and fire 6

The link you provided does have a lot of info. Thank you.

Yes. The barrel pin was eliminated at the same time as the counter-bored cylinder (overlap occurred, of course). Nominally, 1982.

Keep in mind that the L frames were introduced in 1980 and had neither a barrel pin nor a counter-bored cylinder from the beginning. The change on the N frames happened at the same time as the K frames.

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Jack
SWCA #2475, SWHF #318