Hi, below is an excerpt from the "Gibson Electric Serialization, Taken from the 11th Edition Blue Book of Electric Guitars".
Reissues: Early Les Paul reissues produced between the late 1970s and 1993 should have an inked-on serial number on the back of the headstock. The first number should indicate the last year of the production. For example, 8 0358 would be a 1988.
In 1992, the serialization of reissues became standard. The configuration of M YNNN is still in use by Gibson. The M indicates the model code, specifically the last digit of the year of the reissue. The Y indicates the last number of the year of the guitar, and the NNN are the production numbers. For example, 4 8256 indicates a 1954 Les Paul reissue built in 1998 and is the 256th instrument of the year.
http://archive.gibson.com/Files/downloads/bluebook/GibsonElectrics.pdf
When I purchased my Goldtop, it was first mistakenly listed as a 2000 R7, but was then identified as a 1987 Reissue, the reason being the serial numbers were the same. Here's the link to the original ad:
http://www.cascadeguitars.com/cgc222.html
I've been trying to figure out what the above excerpt explains, did Gibson change it's format in 1992 with reissues?