Cistern lever: 3/8" bush means 17mm?

[UserSteve]

I have bought a cistern lever that is described as having 3/8" bush According to the salesperson, the 3/8" refers to the diameter of the large white plastic threaded shaft which passes through the hole in the cistern.

But when I measure the diameter of this large white plastic threaded shaft, it is 17mm. 17mm is NOT 3/8 of an inch! 3/8" is 9mm.

So, what measurement does the 3/8 inch actually refer to?

 

[Basher]

Apparently most cisterns are made for 1/2" bsp thread on the lever, but Ideal Standard use 3/8".
As has been said, the imperial measurements relate to internal bore of the pipe, and the thread o/d's are as below:
if the actual hole in your cistern only measures 3/8" then neither of the standard sizes will fit!