Any pics or detailed threads? I can't seem to get the search functions to work on any of these sites. No matter what I type it comes up with sorry... I'm great at finding that!
It's pretty simple to tell the difference if you have a model 700 to look at. There is a small milled out section on the side of the tang where the safety lever comes past the action.
Look at the small flat behind the safety lever in this picture. That is what you need to do to your 721.