So, before '43 the block system was used universally.... then things get more convoluted.
For example, in 1943 Brunn I (dot) began K98s and used serials (1 to 9999/10000) and just kept going with the alphabet series into '44. That is, in '44 they did not start over with no letter, then a, b... for their block designations. Rather, Brunn I was in the "v" block at the end of '43, and just continued. After "z", they went to "aa", ab" ac... ... aw.
Brunn 1 is the only mfg to use the letter "a" to start all of their two letter blocks designations (others used the convention: aa, bb, cc, dd...).
Finally, Brunn 1 finally did roll back to 1 (presumably) and numbered sequentially into 5 digits, with no alpha. After 100000, they returned to 1a, 2a, 3a....
Here are some examples that can confuse (looking at a "dot 1944 rifles")
- a four digit serial ie., 6478 dot 1944, would actually be a late '44 gun
- Serial 4046 a, "dot 1944" would be an even later gun than above
- 4234 Z, "dot 1944" would be a very early 1944 made gun