I have an Alexander double horn I purchased on eBay. It is a vintage Alexander that looks to be the same design as the model 103, except for the thumb valve mechanism, which is different from the mechanism that has been utilized since at least 1968. Also, it has a slightly smaller bore and seems to weigh less than the Alexander I used to have. The Bell is engraved with “Gebr. Alexander, Mainz, Germany. It has no serial number. Can anyone tell me when this horn might have been manufactured? See attached photos.