I spoke with someone recently at factory on whether or not the new folding CHs
were compatible with mk1s.
Received a hard no on both fitment and individual sale.
I certainly look forward to any progress you make.
Sorry to be the bearer...
Unfortunately, no other lowers are compatible. Nor, as I've found, is the Mk2 compatible with the the mk1 lower.
And unfortunately, I've not found a repr lower after years of hunting.