Tony,
The problem is that if there is something that doesn't work with it, I'm not going to be capable of fixing it for the 21x3 series. And I'm about 99% sure that the plugin will not work with a 21x3 at this time. Because I did a lot of structural changes to better support the 4000 series controllers that are just not compatible with the 2000 series controllers.
The reason I'm not 100% sure is because I don't have a 21x3 for which to test. But just from working on the Galil plugin for many years, my experience tells me 21x3 is a no go now.
The 4000 series is just a much better fit to Mach than the 2000 series ever was. And as good as the Mach/Galil combination was with the 2000 series controls way back when, there really is no comparison to the Mach/Galil combination with the 4000 series. So much so, I dumped all of my 2000 series controllers and bought 4000 series controllers to replace them. Yeah, it was a bit painful. But WELL worth it.
Steve