They say a picture speaks a thousand words. Not a Jimny but enough to get the idea.
The original vacuum system and the manual system operation is much the same, on one the vacuum moves the locking ring, on the other the rotation of the hub dial moves the locking ring.
I assume you are going to disable the vacuum system by blocking the pipes etc.
Not you don't have to fit both to test, but it's easier if you do to see the results. With both locked and the front wheels off the ground when you turn one wheel by hand the other should rotate. With them disengage nothing will rotate.
You should not hear much with the manual system to tell you they are engaged, so just go by the dial position.
Can't help with the screw size sorry, still have my vacuum hubs fitted.
Yokohama Geolanders, Sony head unit, NAUTILUS Air Horn, DRL conversion, Rear cargo space, Elvis Bobblehead, transfer Guard, Indian hanging Elephant, Koni Heavy track dampers, Custom SS exhaust, Voodoo Doll, Adventure Rack with LED ight bar, vintage ERIBA caravan usually attached (yes it's slow)