DECODING Bracelet & Clasp Codes?

[greck]

I've purchased a used 1991 GMT-Master II and am curious about the serial numbers on the clasp and bracelet. The clasp and bracelet are all stamped 78360. One end piece is stamped 501B, and the other is 580. The clasp is also stamped S Z9.

Here are the questions:

1) I've been told that the S Z9 means that the clasp was replaced on service (s) somewhere around August 1998 (Z9)
. Is this correct?

2) Is it common to have different markings on the end pieces, or has one been replaced?

3) Is the 78360 bracelet correct for a 1991 GMT II?

Thanks,
Greg