I think your reply clashed with my edit, but to repeat what I have:
It was originally an A-M 2-litre with the 4 cylinder engine when built in 1948, when it was registered CMF 481, and I believe it raced at Spa in 1949.
In 1953 it was given the aluminium body built on a steel tube frame and designed by Frank Feeley of Lagonda for Paul Jackman, although who did the physical building I haven't been able to find out.
When you ask for which chassis it was built on, do you mean the 2-litre chassis I've mentioned or do you want the actual chassis number? If so I haven't got it...!
Thanks Carnut - we are talking about the same car which you have already pointed out in identifying that it was built by Paul Jackman in 1953/54 and has an Aston Martin engine. It was called a Paul Jackman Special when it was offered for sale some time ago.
So far so good and I'll award you a point for that.
However this was clearly a case of dealer/auctioneer misinformation that is probably now available on the net.
The only part of this car that is Aston Martin is the engine (engine no: SPA/49/3) that was fitted originally to the 2 litre sports (DB1) that was registered UMD 123 that was driven to 11th by Lawrie/Parker in the 1949 Le Mans 24 hr. event. UMD 123 still exists but now has a later DB2 engine.
The chassis of the puzzle car is not Aston Martin. It dates from a well-known prewar sports car (not British).
The aluminium body was built in the UK by x & x Ltd.
The registration number was 481 CMF.
The name of the car is xxxxx (5 letters).