Balance Spring replacement

Hello there. Check another balance if you're uncertain about the depth of the shoe. If it's greater than 1", but less than 1-3/32", then most likely it did indeed just wear down from age and the S385-27 is what you need (if it's 1-3/32" exactly, then of course, this is certainly the balance you need). If the balances are all 1" or slightly less though, then the S380-27 may be the better choice. See the image below for reference. In either case for a stamp option choose 2640. 

Comparison of the S378, S380 and S385 balance shoes.