Sorry I don't have a glock 357 sig but I have a neat one.
This is a Sig 229 SC sports Combo. has two barrels. A 357 Sig with compensator and target back sight plus a 40 cal slide and barrel. The letter is authentication that the serial number is a SIG Sauer P229 Sports Combo. It was maunfactured in December of 2001.
edit if this type thing interests you it is stamped made in Germany