The difference is in the trem, any 7620 will the have the Lo
Pro Edge 7 bridge on it...the 7420 has the cheaper stamped version. Can't comment from personal experience, but most claim it to be far inferior.
The 7620 isn't made anymore, used they go pretty cheap. The 1527 referred above is its kissing cousin.
If I were you, I would buy a used 7620, no question about it.