I was told that the 7421 pickup routes are rounded, and the 7621's are pointed to fit the dimarzio's.
You really can't tell by the stamp on the neck alone, as some 7421's have 7621 necks.
I have owned 4 different 7421's and 3 of them had a 7621 stamped neck.
The color can also tell you the difference. Look at the 7
string section on the homepage of jemsite and read about the models and colors to help you figure out the difference.
Hope that helps a bit =)