I had sent out an XD-M none OSP slide to Powder River Precision to be cut for optics. They make a cut that appears almost identical to the cut on the OSP slides. They also provided the cover plate, an adapter plate of my choosing and the screws.
I found the Powder River Plate wouldn't fit my XD-M Elite OSP slides, it could if you forced it, it needed to be screwed down to lay flat. I also found the Powder River plate also had such close tolerances, that it often hung up trying to place it into the cut, you had to insert perfectly square or push it hard to force it, and get a snap as it popped into place. After Cerakoting my Powder River Precision cut slide and adapter plate, just the Cerakote took it out of tolerances and the plate fit to tight. I had to file the plate down a bit and re-cerakote it. And the filing required to go from not fitting to fitting perfectly, was very little, easily just thousands of an inch....
So, from the posts, it does sound like it might be the same deal. Tolerances so tight, it has to be inserted perfectly square or you hit interference, it might require forcing it a bit and see if it snaps into place.