Since I had been asked to participate, but have never found time yet, I finally did examine my three Amberg rifles fitting to this period:
1. Amberg 1917, serial 5600 q (I think q, but might also be a g?) receiver side marked "3 H7 0" (yes, looks like there is a space between 3 and H7), Letter G on receiver bottom
2. Amberg 1917, serial 1893k, receiver side marked "3E7 0", Letter G on receiver bottom
3. Amberg 1918, serial 437f, receiver side marked "8E8 0". Can't remove H band from rifle since sitting so tight (this rifle surely never had been apart since spiders and plants fell out when I removed the magazine guard), so can't tell on the receiver bottom code
All rifles are all matching numbers, never reworked, reblued, reproofed or anything else - and original WWI configuration.
Edit: just a wild guess (if neither of you have made this one themselves previously) - the German proof post WWII interestingly always coded the year with letters, but leaving out the letter I (maybe because it can be confused quite easily with J if not stamped properly?). Using this system for the months would work too, especially since the higher the serial blocks they only reach up to letter M highest in your listing, which would equal December:
A - 1 (January)
B - 2 (February)
C - 3 (March)
D - 4 (April)
E - 5 (May)
F - 6 (June)
G - 7 (July)
H - 8 (August)
J - 9 (September)
K - 10 (October)
L - 11 (November)
M - 12 (December)
This would for my rifles mean H7 be August 1917, E7 be May 1917 and E8 being May 1918. And it would fit that rifles made in early no letter blocks would use parts made in December from previous year (M6 and M7 coded).