1937 pickup fender spare tire mount

My existing mount where the center of the rim contacts has faint signs of threads in the center hole. I don’t know if it was threaded and stripped out or if a bolt went through it and the faint threads are from the bolt bouncing against the hole from driving down the road. Also, what holds the rim center to the mount? Is it just a large washer or is there a round formed clamp and a bolt goes through and a nut tightens. Any pictures would be appreciated. Thanks in advance for the reply.