19-4 Frame marking/stamps?

[Wallyzworld]

Resent purchase of a clean 6" 19-4. Serial number dates it to 1978 or early 1979. I'm not a fan of the 6" K-frame guns unless it is a K22 or K38, but this has a really smooth double action and very light single action trigger. Not a 3-T gun, has standard hammer/trigger. When I got it home I removed the aftermarket finger groove grips and dug through my box and found some factory square butt grips. My question is the frame markings that I have never seen before. There is an E1, F10 and F15 on the left side of the frame that appear to have been stamped after bluing.
Are these factory numbers stamped after returned for work or somebody else's marks denoting trigger/tuning work?

 

[Doc44]

Standard markings for a S&W made in the 1970s. Stamped by those who fitted, assembled and inspected your 19-4.

Bill