That's the thing with non-A 24 backs.
None of them can have the peep-hole construction, since there is nothing to peep at. Just film to fog.
I will have led you astray, i think (must see what i said exactly in the what-are-these-for thread), by mentioning something about the 220-plugs and 24 backs. The plugs are to plug up the hole in the non-A 12 backs when 220 film is used in those.
And again, in this thread, i shouldn't have said what i said, at least not how i said it: there are no "non-A" 24 backs, except the Automatic ones that do have the type of wind key we see on this one, instead of the folding crank that were put on all A backs later.
The button is still strange...
I had a look, but it looks nothing like the double-exposure switch i mentioned earlier, though it could of course still be something like that, just looking different.
Then again, it could be something else completely.
In short: i don't know.