Viewing the other Keystones on this site it appears
that all the dust proof models have a cover by
the bal.cock.Should all dust proof models have this cover? The one I have s/n 417761,15 jewels does not
and there appears as if there never was one on this
watch.Just curious.
Regards
Roger

Close up of Bal. cock

Dial

Roger, Compare your watch (which is marked "Pat. Dust Proof") to the dustproof (pic below) that passed through my collection. Your plate cuts are different around the area where windowed part attaches to the dial plate. The marking says it should have a cover and if so it may be a variant in the cover plate cutout. Also it looks like yours needs the regulator arm tension spring.

Thanks David.I see what you are talking about but I
do not see how it would be attached as there is no
hole in the base for the screw for holding it on.
Such as yours.
Roger

Look for a hole in the dial plate sort of between the Pallet fork bridge jewel area and the Dial plate O.D.. That is the hole that the big blue (on my example) screw passes through the dust proof cover and attaches it to the dial plate.

David,absolutely nothing there.Oh well,doesn't effect the running.
Roger