1941 Ford serial numbers ran from 18-5,896,295 to 18-6,769,035. 91A is a the model prefix for 1939 Fords (for 1941, it's 11A). There were no VIN's as we know them today. Sounds like someone made up a number for your car. For more information on flathead serial numbers, see
http://www.vanpeltsales.com/FH_web/f...ialnumbers.htm