Found it! I don't regularly get involved in one-off racing cars, especially those built after WWII, but, having established that this car was American built and was named after an American car and its engine of the same name, I decided to give this one a try, and got (very) lucky. The only US car model, that I could think of, whose engine bore the same name as the car was the Lincoln Zephyr. A quick Google of "Lincoln Zephyr Special" led me to a blog, and there it was. According to the blog, this is the: 1941 Lincoln Zephyr Special Racecar, built in the USA, but now under restoration in the UK. I have not found a build date, but it seems to be associated with SCCA racing in the early 1950's.