These seem to be really scarce! Pacific Rim lists two for about $70 each, but that's an awful lot of money. Since I now have in addition to the 50mm f1.4 a 35mm and a 135mm, both of which use the outer bayonet mount, I'd like to be able to safeguard the 50mm when its off the camera. If you have a rear cap to sell at a reasonable price, shoot me a PM.