Interesting thoughts Graham.
Firstly, according to some www. sourced info there was no overal winner declared in 1951. Jock Henderson's account in the Feb 2010 Archive states that there were ....20 places between the winner of the over 1500cc Class and the first AC.....
Re the vent, this is just about discernable when shut in all such photos of OPG 192. I have no recollection of seeing such a vent on any other Two-Litre, but do vagely recall my father talking about one. It would have been very welcome in the summer - provided it wasn't raining!
There are a number of photos of OPG 192 bearing rally number 57 and they all add up to 1951. Except - the original of the start-line photo posted by Barrie is in the Archive, and the white strip pasted along the base of the photo records 1947! That can't be correct - OPG 192 is a 1950 car and there was no RAC rally in 1947!
I cannot find any record of the identity of MPL 340 - the Factory Ledger entries for that period do not include registration numbers. However, features such as the side lamp pintles and windscreen surround place it as a late 1948 or 1949 car.
Part of the conundrum is that the photo of MPL 340 shows three passengers with, possibly, a lady in the front seat. Paul Zetter, the only 1951 AC entry with a crew of 3, is recorded in the Ledger as the first owner of a Sept 1950 car. Therefore, it is almost certainly not Zetter and crew in MPL 340 and unlikely that the photo was taken in 1951.
Your suggestion of 1952 for both the MPL and OPG photos at the hairpin looks to be a more likely explanation. The rally plates on all the photos are RAC. If you can find a full numbered entry list for the 1951 and/or 1952 events, and confirm the limited use of 'The Rest', you would go straight to the top of the class!
John