The T-slot cutter finally arrived, so I did some measuring of it. Thought I'd share this because of my confusion on selecting a cutter. I mentioned above that Enco uses the size of the intended bolt in the T-Nut as the main description on its cutter sizes. The Homier X2 mill specs say it has "7/16 T-Slots". The dimension that matches that, when I measured the table slot dimensions, is the actual throat width, through which the bolt would pass from a T-Nut to hold whatever you have on the table. However, I could not find any T-Slot cutters on Amazon that were specified "7/16". So I ordered the Grizzly "1/2 Inch" model. I checked the Grizzly website to try to determine by what parameter they are measuring 1/2", but found nothing.
Well, the tube their cutter ships in is marked "for 1/2 inch Bolt". Followed by "Grizzly H5912 T-Slot Cutter 1/2-Inch". And the head space width dimension looks to be 0.968”. That is about 0.2" wider than the slots on the X2 table. It's throat width measures 0.505”, so it would not pass between the lands of the slots on the X2 table, which are approx. the 7/16 mentioned above. But milling an initial slot a little wider than 1/2" might allow me to use it on the rotary table tool plate project.
The head space depth looks to be 0.416". I'm not near the mill right now, so unsure of that dimension on its table slots. But I do know the entire depth of each slot below the table surface is 5/8". So if having only 0.250" of material thickness above the T-Nut for it to pull on is a sufficient amount, I could use this cutter to make slots that would accommodate my existing T-Nuts for the mill table, and still be OK within the 0.800" thickness of the plate I made. They would just be a bit wider than needed at the bottom, and also about 1/16 wider at the tops, compared to the mill table slots. Since a bit of wiggle room for indicating stock alignment on a mill job is often needed, I don't think it would be a problem. However, there is always the option of making new T-Nuts specifically for this table, and I could still use 3/8 bolts in them, allowing my current clamping kit to be used on it.
The bottom line seems to be that vendors refer to the clamping bolt diameter when they give a single fractional size for a T-Slot Cutter. Lesson learned. ;)