This is Martin Newey’s superb and very rare 1951 4AC Roadster, seen on the Singer Stand at Powderham Castle in July 2013.
The 4AC model was an experimental run of about 13 cars fitted with a 1200cc, sleeved down version of the SM 1500 engine, ostensibly to be eligible for the 1200cc Class which existed in many competitions around that time. However, when Martin was overhauling his engine he found it was not sleeved, but bored to the nominal 65mm rather than the 73mm of the 1500cc version. (Why go to the expense of sleeving if the bores clean up nicely without?)
The 4AC was more powerful than the 4AB and would have been an attractive little car had it not been overtaken by introduction of the 1500cc 4AD models.