Some minor progress to report...
As noted in my last post, I mailed Tapco with an SKM-192 mechanism which they received yesterday. I also submitted the order for 60 taps on the same day I mailed the mechanism.
I did not hear a response from Tapco until today when I called to request an update. They confirmed the mechanism is indeed M4.2 x 0.35 (which we kind of knew would be the case) and they guarantee the taps will match (this is the part I wanted to see in writing). However, they caveat that the choice of materials we tap into may impact the resulting fit, so be mindful of that. I agreed and asked to formally submit the order. I wanted to do all of that confirmation stuff just due to the sad stories I read from people who purchased these taps in the past and had no recourse when they ended up being too small or a bad fit.
At 1:50pm EST, they responded indicating they would "send an acknowledgement shortly". As of 8:30pm EST, I haven't received that acknowledgement, but I didn't want to wait around until that happened before I shared what updates I had for those wondering what might be going on.
Thanks for your patience and I'll return with updates as they come.