Census Numbers on Amphibious Jeeps

[Ian Grieve]

Can anybody advise the following;

1. When Census Numbers would have changed from the "M" series to the "P" series, eg. M5219692 to P5219692.

2. When Census Numbers would have changed from the "P" series to the following series, eg. P5587101 to 47 XB 55.

3. Does anybody know the rationale behind later type of numbers, ie 47 XB 55?

Ian