You are right about the name. The 1412 and 1421 were double lockers though.
I think the Trem is different to the one found in the 921, 1221 and 1212. In the 1412 you feed the strings from the back of the guitar (its a strange kind of Floyd Rose!) and you dont have to cut the ball ends. It sounds great, but I dont like it!
Well, aside from the cosmetic issue there are other diferences. The 5 piece neck is more stable, the ebony board is a nice feature, and the neckthrough construction provides a very different feel. Plus, the guitar had a kind of semi hollow construction (the called them "tone chambers"). The quality of the fretdressing is also different.
Then come the cosmetical and uniqueness issues.
Dont get me wrong, I like the 921, and the 1221 and 1212. Even the 721 I tried was pretty good for the price.
An RG550 is a very good guitar. But between an RG550 and a jem 10 or a neckthrough J Custom there are other differences besides the cosmetic considerations and details.
Its not just a difference in the finish (flamed top vs regular top). Its a whole different guitar. The only thing they share in common are the pups and the tuners.
Having said that, I can support all your praises about the 921. A pretty good guitar, with the best value in that price range!
Im glad to see that there are a lot of people who care about this guitars