OP - I looked at the picture, and I have an educated guess. It looks like a restruck 28-2 frame used to build a 29-2 to me. I might be wrong...Lord knows it would not be first time. But it look like S&W probably had a previously struck 28-2 frame, but needed to build a 29-2 to fill an order. I know for sure they used whatever they had lying around...that's how you get 357 Model 10's...the frame was made and stamped, but they built a Model 13 out of the Model 10 frame.