The WG3 can be taken as a mix for retailer and the year. I have found no other pieces to compare dates.
On this list below the W designates 1963 and the G is for Gambles.
I don't know how far back this goes , but I know that Murray used a single letter to represent the year since at least 1949. Most of these bikes up until the early 60's had the serial number stamped on the bottom of the bottom bracket. I have seen the letters MOS stamped on most of these. I ASSUME that they stand for Murray of Ohio Supply. This is usually followed by a - and a single letter. That letter will be the year code. After the year code will be a 2 or 3 digit number that stands for the model number. This excludes bikes built for Sears and Gambles(Hiawatha). Sears bikes had a 502 and Gambles bikes had a WG or a WG and a single digit. Sears bikes then had a 2 - 5 digit number afterwards that stood as the model number and was actually used in there catalogs as the part number. Gambles Hiawathas were the same way.
F = 49
G = 50
H = 51
I = 52
J = 53
K = 54
L = 55
M = 56
N = 57
P = 58
R = 59
S = 60
T = 61
U = 62
W = 63
X = 64
Y = 65