I recently put white lacquer fill in paint all over my M-15-3 S&W filling in the stamped areas all over and inside the gun. Looks really nice. I'm pretty sure the gun has been reblued at some point as the S&W logo on the right side is so shallow it won't even fill in on half of it and the "P" and "E" on 38 S.&W. SPECIAL CTG. is too shallow on the bottom of the letters to fill in enough. as is most of "MASS." as in Springfield, MASS.

But anyway, in putting the fill in paint in the stamping inside the crane it clearly revealed the serial # of both the gun itself and the cylinder crane (I didn't know they had separate serial numbers. It also revealed , just under the cylinder crane serial # what looks like @G2 and some illegible numbering or lettering below that. What is @G2?

Sounds like an assembly number perhaps

The G2 is a fitters mark. And S&Ws don't have cranes, Colts do. The S&W nomenclature is yoke.

The number you are referring to as the yoke serial number is the assembly number. You will find the same number under the left stock by the stock pin, and inside the sideplate. It's purpose is to identify the major fitted components of the "assembly", the frame, yoke and sideplate, as belonging together.