500,
Do you still have those two Webleys?
If so it would be good to show a photo of the action face of each rifle with barrels removed, focused into the third bite recess, to highlite the differences in the two systems.
The screw grip action indeed has a true screw grip fastener but the PHV1 does not.
The PHV1 system pushes a wedge between the top of the action frame and the lower surface of the bite on the rib extension so in reality it is no more of a screw grip than is a Holland concealed third bite.
Of course the Holland concealed bite does not have a dolls head where the Webleys do but we are dealing with the bite system here.
The vastly different position of the top lever spindle in the two types gives the game away.
For the PHV1 to be a true screw grip it would need to have its top lever spindle right up on the third bite recess just like the A&WC action.

I'm not suggesting that the PHV1 is inferior to the screw grip in any way, just suggesting that the two systems are very different in the way they achieve the down force on the rib extension bite.
I'd be the last to ever say anything bad about a PHV1.
I shot the hell out of one some time back, which was pretty well used when I got it, and a lot more used when I sold it.
Last I heard it was still going strong and I believe the same gun may have been taken to the big US shoot by one member of the Aussie team a few years back.

(Lang 360 No2. PHV1 extractor)