It's not a RG770DX, because it has a reverse headstock and a rosewood fretboard. The DX's were maple and standard headstocks. The logo on the headstock is wrong, because it's a newer style "chrome" logo with the old-style "U.S.A. Custom" done in white/cream. Looks like someone either repainted an original USA Custom, or took the neck off one and stuck it on another RG body. The USAC's were about the only way to get the rosewood/sharktooth inlay necks w/ a reverse headstock. There's lots of unusual factory guitars out there, but I'd lean towards thinking that it was a combination of parts or a repaint. God knows, I've been wrong before!
Have you ever taken the neck or pickups out to see if there's any different colors in any of the routings?
As for the serial #, I think it means it's an '87 vintage part. Doesn't the first digit after the letter signify the last digit of the year?