Usually, the second letter marks the year:
-Z = 1917
-Y = 1918
-X = 1919
-W = 1920
-V = 1921
In 1920, however, Columbia seems to have introduced yet another bunch of year codes that I have found only on represses (examples are listed on the previous page):
-G = 1920
-J = 1921
These letters can be securely assigned to these dates because the labels on which they appear have two label designs that date to 1920-1922:
Gold Band label with four patents in two lines of unequal length (lower one shorter) (sub-variety v) (used March 1920-Oct. 1922).
This label variant partially overlaps with the
Gold Band Label with Exclusive Artist inside the golden band (used from February 1921-Oct. 1922).
Moreover, as I said in an earlier post, the "GJ" on Columbia A3387 must signify G = July and J = 1921 repressing because this record was originally released only 2 months earlier, in May 1921.
That said, it is strange that I've only ever found the codes
KG (= Nov. 1920) [2x] and
GJ (= July 1921) [9x]
But then, I've found the -V codes, except for AV & BV, even more rarely:
BV (= Feb. 1921) [11x]
EV (= May 1921) [1x]
GV (= Jul. 1921) [1x]
HV (= Aug. 1921) [2x]
I just found an interesting combination of "BP" / "EV" on Columbia A3421 (rec. Feb. & Mar. 22, 1921; released August 1921 per TMW).
EV = May 1921. So BP could have been introduced in June 1921.