Will, I have an old Swannee Whistle which my Grandfather gave me at the age of seven.I played it on stage with his little dance band at Church Halls etc. My whistle is small in comparison to most I have seen being about six inches in overall length and brass plated cap that fits over one end to hold the brass plunger in place.The only ones I have ever seen the cap is just a push fit onto the base of the whistle.If you extend the whistle too far (like my Grandson does) the cap and the plunger are pulled apart.I use vaseline on the plunger, a light oil would do it,but like a trumpet the taste of oil eventually reaches your mouth. There are very few who can play these little instruments well and although I have been playing mine most of my life I still make people wince when they listen to it.
Al