Unfortunately, a previous post got deleted. It is a ruffler. If it is like the one in the second link, you have to move the black part (interposing spring plate) up to have it work on the 15.
A Chinese manual with illustrations of the ruffler
https://www.sil.si.edu/DigitalCollec...ges/image8.htm The one on left is for the 27, but the one on the right is how it needs to be for the 15. I have played with it on the 15. I don't remember if I actually tried it like for the 27, but I know that it did work on a 15 when the spring plate was like the one on the right.
Janey - Neat people never make the exciting discoveries I do.
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