my balancers have part #2420 engraved

Thanks for posting. I will be sure to send Dave your regards. This stamp code only tell us the calibration of the spring within the balance. Can you show us pictures of the nylon fittings on either end of the metal channel? We need to see those fittings to identify what kind or Series of balance you have. From there, we'll do our best to narrow down what you need. Thanks. 

Thanks, that's perfect. This looks like either an S351-25 or S390-25. The only difference between these two balances is the 'depth' of the bottom nylon shoe, so compare both with your old hardware to see what matches best. You will need a stamp option of 2420.