Goffman, here's a photo my 649-3 that shows the location of the model number and s/n on this particular model.
Older models may or may not have the s/n inside the crane, or laser etched inside the topstrap, but they all have them on the gun's butt.
Obviously, I am not too concerned about keeping my s/n secret. But in order to date a gun or find more information about it, the standard way to id the gun's s/n range is as stated above, for example mine would be CDD71xx.
hth