Any chance that you'll add that list of specific layouts and/or a diagram of the mounting plate on the OMNI2 webpage?
Not trying to be pushy here, just trying to clear things up so your customers can make an informed decision. 
I will try to figure out the best way to accomplish this in a unified manner. The funny thing is that I just had someone tell me that I should indicate specific compatibility by type on an item where the dimensions were stated. On one hand, those who know how the dimensions relate to the models find it more convenient to have named compatibility, while others who don't, look for specific dimensions. Then there is the third group who just make assumptions, and disregard both. Perhaps it is time for an FAQ, or table showing compatibility, along with some dimensional diagrams at the site. This should cover groups 1 and 2. Not much I can do for group 3

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But for the sake of the neophytes, I'll add some info here: There are two major categories. The first is the WICO, HAPP, X-Arcade, IL type. This is what is likely on your "old-school" US-built machine, as it was the original de-facto standard. Then there is what I call the "Japanese" type. Sanwa, Seimitsu, Zippyy, Spitfire (and similar) use this. These are narrower, and have the obvious "angled, rounded-end slots". These two types are not compatible with one another for mounting, without adapters or modifications to existing panels. The OMNI2, and I assume the JLW (no direct experience, as we don't carry them), can accommodate both patterns.
Then there are the "oddballs". A number of original machines, usually of Japanese origin, used proprietary controllers and aren't directly compatible with much of anything other than the controller they shipped with. There is also the "Korean Standard" and newer designs (the Leaf-Pro, for example), which adhere to nothing from the past.
So my suggestion for folks is not to get in a hurry. Whether starting fresh, replacing a controller because it is broken and worn out, or looking for something better, it's a good idea to do the research to find the control with the features you
really want, rather than just what fits, and make accommodations for it. Wait for it to arrive, take the plate off and use it as a mounting template so the work is performed only once and works the first time. Outside of perfect dimensions and a CNC machine, there's no substitute for having the actual item in-hand to use as a reference.