When did Smith stop putting the gun serial number on the cylinder and the back of the ejector star? Hope this is the right spot for the question. SN on the cylinder is 163250 followed by an S and a diamond shape.
In May 1957 S&W stopped putting the butt serial number on the rear surface of the cylinder and on the barrel when the Soft Fitting operation was discontinued. However, many guns were in the pipeline and shipped after this time. Your revolver most likely dates 1956 and was made before this change was made. S&W continued to number the extractor well into the 70s.
I two model 66's, one is a 2K with only a V stamped on the rear of the cylinder, the other is a newer 9K that has the complete serial # stamped on the rear of the cylinder along with a Diamond and the V.
The 9 K was sent back to the factory for a re-barrel back in the late 1970's
If I remember correctly the serial number & Diamond were added to the rear of the cylinder at that time.
So, If I am correct there may be other S&W's out there with the serial # on the cylinder that do not fit within the dating system of these stampings by the factory.
I have read that S with a diamond probably indicates the gun was reworked or repaired at S&W. I have an M&P 38 Special (pre Model 10) so marked that also shows the RS stamp and date on the frame indicating it was refinished by S&W. Mine also has the S diamond under the barrel.
It is on the Acess/Misc for sale section. http://smith-wessonforum.com/access.../390616-2-k-frame-barrel-cylinder-38-spl.html