Great post! You made me curious so I pulled out my 66-2, #175Kxxx. It's a 6" model given to me by my dad some years ago. Unfired at the time, but we were able to fix that condition together. So anyways, mine does have the trigger stop. Inside the box was this printed notice:
"The slot located in the Frame directly behind the Trigger is to accommodate a Trigger Stop, which eliminates excessive Trigger travel in serious single-action shooting. Such a Trigger Stop is not a standard part in the Model 15, 19, 66, and 67 but is available as an optional accessory."