For a long time I didn't think any serial numbers between 000000 and 050000 existed and that's why they're not covered by the serial number decoder. A few has however popped up over the last year and they don't really fit in.
Here's the options we have:
The serial numbers rolled over from 0500 to 010000. In that case the earliest numbers are 5 months off and production started as early as June 77.
4 digit numbers and 6 digit numbers were used simultaneously during the transition period. This way 010000 was produced at the same time 0100 was made. This is how the serial number decoder works today.