Another rabbit hole chase. Looking for pictures of what is missing from this whistling tender. Sound chamber and relay. I seem to recall that AF was infringing on the Lionel patent with this tender. Better yet would be parts sitting in your junk box.
The tender is a prewar chugger tender, not a whistling tender. It is missing the cylinder and its support. The plunger (that you do have) is driven in an oscillatory inside the cylinder. Contact Doug Peck, who may have the necessary bits.
Hope this helps.
That is a chugger unit for American Flyer 3/16 pre war O Tenders. The cast cylinders are not available as reproductions. You'll need to find a junker tender or someone selling the whole unit complete. Be sure to carefully inspect the casting for cracking from zinc pest. Be sure there is a small tabbed piece attached to the outside of the cylinder that hangs inside the square hole on the side as it is the sound maker and is easily broken off. The leather piston can be softened by soaking for a few hours in boot oil. Doug has complete units available. The tender that ruffled Lionel's feathers was the remote whistle tender paired with the 314AW K5 in S gauge.
Your missing the chamber which is diecast and prone to the old zinc pest. No parts source, I've considered making one out of a copper or other metal tube in the past.
Doug Peck has an online repair clinic for this on his Port lines Hobbies website. Look at the top of his site for the Clinics spot to hit.