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I recently bought an old sterling euphonium from a buddy and it is missing the 4th valve lock. does anyone know where i would be able to purchase such a part?

Maybe contact Custom Tubas? I think they are the U.S. Dealer for Sterling, and can probably get one. I don't see why they would make any major changes to the design if it worked.

Most techs could also probably fabricate one relatively easily. It's pretty much just a bent piece of brass with some cork under it.

For reference: the Valve Lock on my sterling. The silver plate is pretty much gone, and I really suspect it may be a replacement itself.

My Mack horn is the first euph I've ever owned that had a fourth valve lock. I never use it, mostly because I always forget.