Hi Al -
Welcome to S1 ownership. This is the place to get the information you need on the S1.
You are right - there are too many numbers in the Vin he sent you and they don't start with AC for S1 cars, but the picture of the plate shows the car was built in Nov, 1977 and the etched numbers below it refers to the order of the car build - #516, H = a federal (US) car.
For comparison purposes, my Vin No. is 77010116H. Date of manufacture is '77, January 01, car no 116 in the production line, all those numbers started with 100, so mine is the 17th made, H is for Federal (US) version.
The Vin No should be on a plate in the bottom of the windshield on the drivers side. Ask for a picture of that. Once you have the correct Vin No., send it in an email to Andy Graham at Lotus (agraham@lotuscars.com) and he'll send you a letter and a Certificate of Provenance which includes the engine No and transmission No plus the color for that car as built. Worth doing before you buy.
If we can help further, just ask. I'm in Connecticut, where are you?
Cheers,
Bob