To the small number of members, (Mr MCarver for example) that have some "6th sense" ability to state as "fact" without explanation that they have the answer.
I am sorry if I offended you with my rather matter-of-fact statement about your Model 19-3, which you indicated was made in 1976. I could be wrong, but do not think so. I think it unlikely that your revolver is a mismarked Model 66.
One reason, obvious from your pictures, is the lack of the red ramp front sight. Not definitive, for sure, but a factor I considered. The rear sight assembly appears to be blue or black, but that would have been easy - that part was simply not refinished at the time, betting the revolver was originally blue.
Model 66s made in 1976 had red ramps, and I do not see, on your pictures, the red ramp, nor do I see the dovetail cut where one might have been. Again, we are all guessing, and we are all trying to help. I sincerely apologize if I offended you.
I hope your revolver is rare, as it is nice to find a rare one from time to time, but, and I mean this in the nicest way, the evidence shown so far, does not lead me to the belief that it is actually a Model 66.
I will be more than happy to admit, in advance, that I am wrong, if the correct markings for stainless are shown as Bill suggested, or if a factory letter says it is.
Another suggestion is to call S&W and ask customer service to enter you serial number in the computer to see what they can tell you about it.
Good luck, and my sincere apologies. Again, I was merely trying to be matter-of-fact, and was not trying to offend you or your revolver.
Shawn