My guess is that it is a prototype Custom that was built before the P-15. It is engraved "Olds Custom Model" on the 2-piece yellow brass bell. There is nothing on the bell that would indicate it was another model bell at some other point and the original engraving was buffed out or somehow hidden by the new engraving. At the threaded part of the bell where the slide attaches, the word "TESTING" is stamped in all caps.
My Custom has:
1) ...a rose brass ferrule between the gooseneck and tuning slide receiver. Supers are all nickel-silver. I can't recall if any small bore Olds horns had rose-brass ferrules.
2) ...a one-piece flared brace at the handslide cork barrels. Supers are 3-piece.
3) ...a straight tubular nickel handslide brace with oval-shaped ends. Ambassadors have a similar handslide brace, but the oval-shaped ends are squared-off on the sides. Supers have flared 3-piece braces. Standards have round 3-piece braces.
4) ...yellow-brass outer slide tubes with a nickel-silver crook and no oversleeves. Supers have nickel-silver outer tubes and crook. Standards have nickel-silver oversleeves with brass outers and crook.
5) ...a press-fit leadpipe.
6) ...rose-brass rings at the very end of the yellow brass outer slide tubes. I can't recall if any Olds small bore horns had this feature.
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IMHO, my horn is not a P-15, a Super, a Standard or an Ambassador. There are parts on this horn not found on any other small bore Olds (to my knowledge). It is as the engraving states, an "Olds Custom Model". 1 of 1