If you don't know the make or the model it doesn't matter at all. In my last movie I wrote more than 20 "unknown". The site is for that.
By the way, for the last one i would go for a 1939 Plymouth De Luxe P-8 (only luck, believe me)

http://www.prewarcar.com/show_prewar_car.asp?car_id=47571
http://www.imcdb.org/vehicle_40669-Plymouth-De-Luxe-P-8-1939.html



But if we aren't completely sure, the best way is write "unknown" or "Plymouth unknown". For sure later will come in our help some more users.