Originally Posted by
carbogast
I have a York euphonium with this feature. The extra slide serves to lengthen the main tuning slide and convert it from high pitch to low pitch. York had a patent on this, and the slide is likely to be stamped with a 1909 patent pending label. The 1st and 3rd slides should have a mark showing the nominal position for low pitch. The York euphoniums with this feature used two separate slides for the 2nd valve. Unfortunately, "low pitch" was not A-440, but something a bit lower.
Edit: As I look more closely, it isn't the same scheme as is used on my York. However, I think the purpose may be similar. In your instance, I would expect that there were originally two slides, one for high and the other for low.
Another edit: I get it it now, the loop is not adjustable... my above comments aren't relevant (:X