The S&W 547 was made in 3" and 4". It used a K frame, same as the model 10, 13, etc. The biggest 2 differences were the extractor rod/star and the firing pin. As the 9 is rimless, the 547 did not use clips, therefore needed a way to extract the empty brass.
It used little springs or fingers that retracted into the extractor rod area. You would extract the same way as a normal rimmed round, ie. 38spl but when you would push on the extractor rod the 6 little springs would extend under the rim of the 9mm brass. Not the greatest of systems as it was susceptible to dirt.
The firing pin was an internal firing pin, similar to a colt python or any of the S&W J frame centennial models, ie. 640, 940, etc.
As a note, a lot of PPC shooters converted the old 547 to 38spl, as the myth was you could lighten the trigger pull more as the internal firing pin was supposed to give a more positive ignition.
