You are buying two Clarostat 0-500 ohm potentiometers. Model 210-73. I pulled from a used piece of 1950’5 equipment and they have date codes 140330, 334, 423. They all read 500 ohm on my VOM, the needle tended to jump when the shaft was turned. They probably need to be cleaned. They got smoother when they were worked a bit. Used.