741603 are probably not the six digits that you had in mind, but here are a couple of my favorites -- 74033 and 741603:

following your "key", we see:

• the XX = 74, telling us that the movement is the Valjoux 7740;
• on the Autavia, the "1" tells us that the movement is not simply the 7740, but the GMT variation of that movement, so we open with 741
• then we see the "3" for the Monaco and the "6" for the Autavia, as we have come to expect, and
• the final "3" tells us that these are in steel cases
• seems that the "0" is just for spacing purposes??

: Thanks Jeff. In a flash of inspiration last evening, I realised an
: assumption about what would be carried over from the 4 digit
: code was preventing me from seeing something obvious in the 6
: digit codes. I now think I understand those too, bar a couple of
: anomalies, so expect a follow-up posting today once I have
: collated the information.