Dr. Ir. (Achmed) Soekarno, blacksmiths

5 digit serial # with double Block letters
serial # 00001 - not higher than 29602: Block letters "MM"
serial # 29603 (lowest known 48587) - 999999: Block letters "MA"
Subsequently, the 1st and 2nd letters increase alphabetically.
In line with this progression the following Block letters are formed: NB, OC, PD

6 digit serial # with double Block letters
The transition from 5 to 6 digit serial # happens in the "PD" Block letter combination.
Subsequently, the 1st and 2nd letters increase alphabetically.
In line with this progression the following Block letters are formed: QE, RF, SG, TH, UI (highest known number: 193218 UI)

It cannot be ruled out that more Block letter combinations do exist (e.g. VJ and WK), but they have never been seen so far.