The protocols used are based on the car type, the material used, the number assigned in the page and the angle of the photo. eg. Hudson, Pressed Aluminium, 6, A . ie Hudson_PA_6_A.

The common  symbols used are : CA= Cast Aluminum; CB = Cast Brass; Fe = Iron; NP = Nickel Platted; PA = Pressed Aluminum; PB = Pressed Brass; PS = Pressed Steel

 Thus NPPS = Nickel Plated Pressed Steel;  NPPB= Nickel Plated Pressed Brass

A = top photo; B= side photo and C= underneath shot.

To determine the number, use the search function in gallery for the car type. This will bring up a view of the caps. Then use the next sequentual number from the type of cap