ARCA-style plates: What're the screws for?

The plate which shipped with my Sirui ballhead included a large QR plate
which I don't intend to use, while the others I bought are smaller.
The difference is the plate undersides: The original is free of hardware
other than the screw for camera/lens, with the others having two
hex-head screws each:

Usually they would be slide stops to keep the camera from sliding out of the clamp if the clamp came loose. Usually this is something you see on lens plates to keep them from sliding out, I don't know why they'd be on a camera plate as the ballheads usually have a pin to catch the camera plate, rather than putting the pin in the plate.

Based upon the direction that you prefer for your camera when you turn it on its side for Portrait more (CW vs CCW) you remove one of the screws.
The one that remains allows the camera to slide into the QR in the partially released state and STOP without continuing to slide downward and fall out before you locked it in place.
I don't use that feature myself.

Wilt wrote in post #18563887
Based upon the direction that you prefer for your camera when you turn it on its side for Portrait more (CW vs CCW) you remove one of the screws.
The one that remains allows the camera to slide into the QR in the partially released state and STOP without continuing to slide downward and fall out before you locked it in place.
I don't use that feature myself.

Well, this makes some sense; I hadn't thought of it.
Doesn't seem to apply to me, either, as I mount the camera before adjusting the tripod.

SkedAddled wrote in post #18563895
Well, this makes some sense; I hadn't thought of it.
Doesn't seem to apply to me, either, as I mount the camera before adjusting the tripod.

One viewpoint is that the screw stops the camera at the 'right place' -- even horizontally, you need not try to center the plate in the QR, you merely slide it in until it stops and then lock the QR
Since I prefer to open fully the QR to insert/release the plate, sliding in the plate from the side so it then stops is not necessary

Wilt wrote in post #18563898
One viewpoint is that the screw stops the camera at the 'right place' -- even horizontally, you need not try to center the plate in the QR, you merely slide it in until it stops and then lock the QR
Since I prefer to open fully the QR to insert/release the plate, sliding in the plate from the side so it then stops is not necessary

Another reasonable rationale; thanks.
The stop-pin keeps my gear from heading towards destruction, as I slide the plate in
while holding firmly, so another reason I don't want the screws.

Interestingly enlightening explanations all the same, so thanks again, Wilt.

On a RRS leaver clamp if you open the leaver half way you can adjust the position of the plate, this is very useful for nodal slides to get them positioned perfectly. The screws physically stop the plate sliding off the clamp.

You might not think you need it, however the first time it saves your camera from falling from a height you will thank it for being there. If you really don't want it they are trivial to remove.

hoskoau wrote in post #18563934
On a RRS leaver clamp if you open the leaver half way you can adjust the position of the plate, this is very useful for nodal slides to get them positioned perfectly. The screws physically stop the plate sliding off the clamp.

Took me a moment on this one, but I think I understand.

Also does not apply to me; I slide the plate away from myself into the head to attach
and towards myself to release. And again, I make a firm mount prior to any
tripod head adjustments.

Actually, those are safety stops. They keep the camera from sliding out if the clamp comes loose.

bigVinnie wrote in post #18565292
Actually, those are safety stops. They keep the camera from sliding out if the clamp comes loose.

You're right.
The Sirui head has a built-in safety stop however, making these screws unnecessarily
redundant for my own useage. The built-in stop engages the underside of the plate.

Far as I know the Sirui clamps are the only ones that have that feature.

bigVinnie wrote in post #18565467
Far as I know the Sirui clamps are the only ones that have that feature.

Seems to be a big plus in favor of Sirui.
I know nothing more about this, so somebody more knowledgeable may chime in
with further information.

bigVinnie wrote in post #18565467
Far as I know the Sirui clamps are the only ones that have that feature.

If you're referring to the safety stop built into a ballhead clamp then there are plenty of heads that have them. Sirui isn't the only one, or the first.

I have on occasion loosened the wrong knob, ie the clamp instead of ball head. I have never had an accident, always realised quickly, but those screws provide a backup to stupid mistakes like that.