well they are pretty close being that they are both fixed bridges. Now if we were talking the 7620
and 7420 models, there would be a world of difference. 7620 would have Lo-pro and 7420 would have Lo-TRS II. I believe the 7621
has Dimarzio Blaze pups, whereas the 7421 would have V7/V8 series pups. Both will have the same tuners and necks are the early 74XX's would be the same as the 76XX's.
You may want to swap pup's on a 7421 as they aren't the best, whereas the 7621 would have Dimarzio's.
And if I'm not mistaken the 7421 would be made in Korea, the 7621 is MIJ. I'm not real familiar with the fixed bridges. However, when it came to the models with floating trems, I'm very familiar with them.