BG = Bernie Grundman. http://www.berniegrundmanmastering.com/

CB = Chris Bellman. http://www.berniegrundmanmastering.com/

GM = George Marino. Formally an engineer for Sterling Sound. The late George Marino ended his career at Sterling Sound as a senior engineer. http://sterling-sound.com/engineers/george-marino/

RJ = Ray Janos Sterling Sound. http://sterling-sound.com/contact/

RL = Robert Ludwig. http://www.gatewaymastering.com/mastering_bobludwig.asp

WG = Wes Garland. Followed by /NRP. Wes Garland and  Nashville Record Productions http://nrpmastering.com/

Optimal Matrix scheme:

(Single letter format identifier, followed by single character year identifier, followed by a 5 digit numeral code, followed by an appended two digit disc numbering).

X is the format identifier:
"A" = CDs
"B" = Vinyl records
"D" = DVDs
"E" = Blu-ray Discs

Y is the manufacturing year identifier, since 2010 a single letter (A to Z):
A = 2010
B = 2011
C = 2012
D = 2013
E = 2014
F = 2015
G = 2016

After the 5 digit numeral code, the 2 digit disc numbering (ZZ) can be found, which starts at "01".
On multi disc releases the last digit usually incrementally rises by 1 for each disc. (Example: Disc 1 = 01, Disc 2 = 02, etc.)