Upload & Sell: On
I I have now looked at using my old Really Right Stuff L plate off of my 1D4 on my new 1DX, and it is close, but does not fit. The 1DX has a very similar base, but the front to back width is slightly larger, and the location of the tripod mounting threads is in a slightly different position. If you hold the old L plate on by hand, it rests with the rounded edges in contact, and the main flat part of the plate is about 0.07 inches above base of the camera. Also, the camera threads are about 0.05 inches out of line with the slots in the L plate where the mounting screw resides. If you tried to pad the gap between the camera and the plate, and filed out the slot wider to allow the screw to go in, you might get the old plate to function, but in that position, the clearance for the battery to be removed is around zero, so it will either be blocked or will rub hard as it is removed or replaced.
In addition, the rubber covers of the 1DX are of a new design. They are wider from front to back, and are hinged along one edge rather than just attached by the small rubber strands used on the 1D4. That means they stick out further when open at 90 degrees, and can not easily be mushed down out of the way. But the new covers can be folded open 180 degrees on their “hinge”, which means that for using the remote, the L plate can be slide against the 180 degree open rubber cover, and hide it away under there. And with the cover captured at 180 degrees, the position of the plate is just right for the Canon remote connector to clear an AS quick release clamp. The existing first slot in the old L plate is thus long enough to allow the required position for a remote attached to be attached.
The second slot in the old L plate will still be needed to allow attachment of connectors that exist along the rear group on the camera, which are the USB, HDMI, and Ethernet connectors. Note that these also all have the new style covers, which look more durable and better sealed. Note that the new covers sick out about 0.75 inches when open 90 degrees, but also can be folded back to 180 degrees open if needed. Obviously if both the front group and the lower rear group rubber doors need to be open at the same time, neither can be flopped back 180 degrees, so the mounting slot provisions need to accommodate that configuration with them both at 90 degrees. And another thing about the rubber cover doors is that you can have the front lower cover for the remote control and the upper rear group cover for the Ethernet both folded back 180 degrees, because they are offset just enough to do that, and that is likely a common configuration for remote operation.
A design change that would likely be good for RRS to incorporate is to make the two slots in to one longer one. That would allow faster adjustment between the two general positions because the screw would not need to be removed and replaced. It also would allow any position to be set, which would include intermediate positions if you are using right angle connectors that do not protrude as far, and may have nice small flexible cable on them. The current missing positions on the 1D4 plate, because of having two slots, is in just the wrong place for using the smaller right angle connectors. One continuous slot just seems like the way to go.
Another design change that could be included would be to provide for a cable routing path on the inside of the slide leg of the L plate, and an end opening, that could help any rear group cables that are in use along there to be contained and routed "out the top", and keep them from flopping to the front or rear and getting in the way of the L plate being clamped. It would be possible to even include some edge tabs that could capture the cables, and that would require you to bend the cables in to an S shape in order to pop the cables in the routing path.
I do hope that Really Right Stuff stays with a tried and true design that is very similar to their existing 1D4 L plate, and that it "fits like a glove" when installed. Add new features as required, but stay with what has worked from the past. The initial issues with the fit of 5D3 plate were of concern to me, and I also do not like the idea of any sort of two part plate that is convertible. Please RRS, design and build a perfect one piece L plate for this very nice camera. I will gladly buy a second bottom only plate if I need that. Also remember that this camera is very suited to remote use, and that requires mounting, so the best allowance for cables is important, and remote cameras may need the quick changing of orientation that a good L plate provides.