Meet the Colt Magnum Carry, a factory stainless steel .357 made on the post-1972 DS-size (D-frame), the same size frame as the Diamondback.
They are rare, made only for one year (1998 through 1999), as a .357 Magnum/.38 Special revolver.
The first batch made carried the roll stamp, "1st Edition" on the barrel next to, "Magnum Carry".
They were issued in a Colt Custom Shop box.
SN's start with prefix SYxxxx.
Diamondbacks, however, were blued carbon steel & had a vent-ribbed bbl, and AFAIK were never factory chambered for the .357 - so I'm also of the opinion that somone most likely had it, looked at (or heard of) the .357 Magnum Carry, and did (or had done) a rechamber/remark job.