I purchased a Midwest Industries picatinny optics rail for my MKE AP5-P clone.  It seems that the rail was slightly out of spec and I could not mount it to the receiver, after much effort.  A buddy of mine who has a few MP5s also took a look and he had no more luck than I did.  I have since sent the rail back and received a refund.  I would like to purchase another, as surely I wouldn't receive two rails both out of spec, especially from a company like MI.  My question (both for anybody at MI or anyone who has also purchased this rail for their MKE AP5-P) is has anybody else had this problem?  Its got me wondering if I just got a random lemon with the rail I purchased or if there is an issue with putting this rail on an an MKE clone (specifically the short pistol type).  I would certainly appreciate anybody who has experience here to set me straight.  Thanks.

Link to rail purchased:
https://midwestindustriesinc.com/mi-hk-mp5-and-clones-top-picatinny-rail/

I have one of these and it worked fine. I completely unscrewed and removed all four angled "nuts" first, dropped it on the gun and then slid those nuts back into place and tightened them down. You do have to tighten them evenly and make sure the rail stays fairly level.

Yeah, I tried the same method and it was simply too tight.  I could get three of the nuts/wedges in place but not the 4th.  I think it was just too tight to get all four into place.  Probably just a machining error, but what do I know.

I have this issue too but with a full size MKE AP5. The mount itself fits okay but is too narrow for the clamp pieces to fit underneath. I had to tap them in with a punch and even then the clamps sat too low for the screws to catch in their threads. I messed with it so much I flattened out the threads on the screws and they'll need to be replaced.

All I can think is the MKE receiver is wider than the spec used to make these optic mounts.

Interesting, just to make sure are you putting the clamp "nuts" in the right orientation?  The corner edge with the 45-degree cut should be inward (toward the receiver) and facing down.  This quick paint diagram shows how they should go.  The black is the receiver viewed from the back, the red is the body of the MI rail, the blue is the clamping nuts, and the grey is the screws.

You also need to remove all the screws and clamp nuts first before you try to slip the rail onto the gun, or else it won't get over the clamping bosses on the receiver.  Then once it's on you have to carefully slip each nut into that space and wiggle it around until it's positioned for the screw to grab into the threaded hole and tighten.

Interesting, I just got the same mount for my AP5 and I have the same issue. I can get three in but the one behind the ejection port is way too tight to fit. I even tried to file down the back side of the tab and it still won’t fit.

I did all of that. I took the clamps and screws all the way off, the put the mount over the receiver that went great. With the clamps in the correct orientation, I could barely fit one or two of the clamps in and it was difficult to get them seated high enough to get the screws to catch the threads. After getting one side started it was impossible to get the other side. The gap was just too narrow. I tapped the clamps in with a punch and hammer but they were too low for the screws to reach.

The width of the receiver plus the width of the clamps is wider than the gap in the mount.