The Chiltern "Parliamentary" service to/from Paddington via the New North Main Line also follows the rule:
2V27 1057 South Ruislip - London Paddington
2M29 1136 London Paddington - High Wycombe
Traditionally there was always a "gap" in the numbering of the relevant trains as well, so if the service was diverted to Marylebone (e.g. because no Guard was available), they could be renumbered with an appropriate 2Hxx/2Wxx ID instead.
2V27 1057 South Ruislip - London Paddington
2M29 1136 London Paddington - High Wycombe
Traditionally there was always a "gap" in the numbering of the relevant trains as well, so if the service was diverted to Marylebone (e.g. because no Guard was available), they could be renumbered with an appropriate 2Hxx/2Wxx ID instead.