Possibly the oldest hand cannon dated 1346

[fernando]

What happens if we mess with numbers, Marcus ? We find out this is no linear subject !
If we assume that the date you mention to be that in the Landshut stone is 1407, variations between Indo-Arabic numerals and the contemporary inscription must be considered; in that some digits suffered a "rotation", for a start.
The number you point out as a 7 used to be a 8, and the number 4 was earlier in the inverted position. So we may face in this case an European version of Arab numerals. On the other hand, the marriage of Roman number M with those of another origin is still a riddle ... for me, that certainly not for numerologists.

.