Just picked this up and was wondering how it can be in such great shape. When did Olds mark mps with "Los Angeles"?

Based on the shape and the markings being on the underside of the rim, I'd say that's a very early example of the classic Olds shape - likely from the mid-1930's. No reason a mouthpiece that old can't be in good shape if it was properly cared for.

Any chance it could be a worn or poorly stamped "8" rather than a "3"? That would fit with Olds' sizing system of that era (and 8 was the standard size).

It is a 3. I've never seen the number sideways like that. The seller claimed it was from the 30s. I guess it's either OSS or has been replated. Looks brand new. One other thing is the fit into the leadpipe isn't tight like my 50s Olds #3s ( I've used it it my '46 Standard. Haven't tried my other LA Olds horns yet).