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I recently received a Kanstul 80 F tuba and was curious where I could find the serial number on it. Where the bell and bottom bow meet the number 65 is engraved there, 65 is also engraved into the outer tubing of the 3rd and 4th tuning slides. I just received the horn yesterday and haven't had that much time to look it over for little things like that.

Bump, looked the horn over and haven't found any other numbers, isn't the serial number usually found on the 2nd valve casing?

Depending on the maker, the serial number can be on the receiver, the second valve casing, or on the bell somewhere. I am not sure where it is for Kanstul, but I'm sure if you contacted Baltimore Brass they could help you out as they regularly stock these horns.