This is a replacement diaphragm that operates the second stage on a Solex 44 PHH carburetor.
Please note that not every Solex PHH is equipped with such a diaphragm.
Also, there are two version of this diaphragm:
1) A "fixed" version which is not adjustable and consequently has no thread on the back
2) An "adjustable" version with thread on back which allows adjusting the operating rod
This diaphragm here is that second version, the adjustable one (see photo).
Required quantity per carburettor: 1
This diaphragm is not always shown in the drawing.