The funny thing is they sent the letters to FFLs inquiring about unfinished frames. But since they are not firearms, they can be sold by anyone so their can be hundreds of thousands floating around elsewhere.
I mean, the first thing that comes to mind in the 80% parts. But really this can apply to an aluminum block with slide grooves in either side, for a 1911 or 2011 model.
The squishy term ‘readily’ is cooked up by ATF to cover whatever they want. It's definition is the definition of ambiguity.