The early Live Free Armory receivers had the slant cut, and ran both thread pitches - some were 18 pitch and some were 16 pitch - so you even had to be careful with those, for handguard/barrel nut choice. Now, all the LFAs are no slant-DPMS LR-308 based. I have an early LFA set, and testify to that, and provide pics.
That is exactly "splitting hairs" and I'll give you the direct answer: There IS NOT a mil-spec on large-frame ARs. Nothing to "follow", for any manufacturer, so any manufacturer out there can do whatever they want, whenever they want.
As consumers, we have a giant ball of trash to wade through, every single time a new manufacturer releases a new rifle. Gotta figure out WTF they did with this thing.