Saw this on a local site, one of the pictures shows it marked 66-2. Far as I know every Model 66 was chambered in 357. I wasn't the buyer.

But the 681 was an L-frame, basically a fixed sight 686. Here's a thread regarding the 681 chambered in 38 Special.Smith once did a special run for the railroad police in some state. .38spl only in a 681, I think?
No clue about other models.