Mine has a bone air button and stap end fixings.
Interesting. It would appear that the move from bone to metal, for the air button, came during 1918. From your photo, it looks like the bone button is of standard size. I had a 40 key Wheatstone (No. 26250 from 1914); this one had a bone air button, but a larger size. I would guess that Wheatstone firstly reduced the size of this button, then decided to replace it with a standard metal one.
I'd be keen to know whether other Wheatstone owners, with instruments from this period, can shed any further light.