I poked around some...
MLOK was invented by Magpul. They do not show a patent on the rail, and there are notes saying the rail pattern is free use. Magpul licenses the use of the MLOK name. So it sounds like anyone can make a rail using the slot pattern, but must pay Magpul if they want to identify the pattern as MLOK.
Magpul does have patents on every possible thing you can attach to that rail, and the little nuts that hold those things onto the rail. And they have trademarked every name of everything. Patents run out in 22 years, and trademarks can be renewed for as long as they are in use. So that is a long-game approach to making money.
Most of the drawings for the rail show only the length and width of the slots, shape of the corners, and spacing between. Then the back side of whatever attaches and the little nuts. The only drawing I came across that has a section marked with the little arrows that point to the side and the letter "B", does not have the drawing of the section included.
There are drawings of rails from different makers of floating hand guards, and they ignore rail thickness or width of the hand guard (beyond one saying 1.5 inches), and seem most concerned with a spec for the air gap between the barrel and hand guard, and the depth of the slot for the gas assembly above the barrel.
The standard bolts for the MLOK rail covers, sling mounts, and Picatinny rails to mount to the MLOK rails seem to be 3/8 inch, and too long for the XCR. 5/16 would be better, but apparently not made in low socket head cap screws, and I wanted a fully coated bolt that would not rust on the end like if I sanded it with the belt sander. I ended up with quarter inch.
So, it appears that the XCR has thinner than standard rail cross sections, that leave more bolt sticking out than other rifles and hand guards. And a minimum clearance between the inside of the hand guard and the barrel.
Whatever the end of the hand guard width discussion, plan on buying a bunch of 1/4 inch long socket head cap screws to replace all the 3/8 inch long bolts that come with all of your MLOK accessories.