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I recently came across an old Eb Conn upright with 3 pistons Serial number research puts it at being a 1914 vintage.

It also has a 6 inche removable extention in the main tuning slide, betweeen the leadpipe and the valve cluster. Would the purpose of this slide be to switch between high and low pitch tuning? Is this common to have an arrangment like this?

After a few dents are pounded out and valves rebuilt, I think this will be a nice little horn. This will be my first venture into the world of Eb since "...The Beginning..." in grade schoool.

It is if by not using it, you're in approximaely E natural,
and by reinstalling it you're in E flat.

Yes, that was a very common way for makers to provide horns that could be used with both High and Low pitch. Horns of all lengths had similar arangements if they were made to play in both.

The valve slides are probably a little short if it was made to play in HP. With a Conn, you may also find engravings on each valve slide as to how far to pull it out as a starting point for LP.

In older vintage horns, you may well find such an arrangement to bring it down to A=440. Lots of town bands had members with the older high pitch horns, and didn't want to shell out the money to switch, but by buying a horn that could do both, you could put it off for a while. By the 20's, everything pretty much was made by default in low pitch.