The problem with 'x%' completed receivers is that there is no approved guidebook for what can or can't be completed for it to be considered a firearm, other than it's ability to discharge a round.
One.
Doesn't matter if it blows up, if it fired one round, even if the RCMP had to duct tape things together to make it work. They don't often play fair.
One could make a '40%' lower that wouldn't accept a grip, or a mag, or even have the cross pin holes drilled for the upper, but could still discharge a round.
Just be careful of which 80% is completed when you purchase.
I have seen people trying to sell unfinished lowers with the auto sear cut in the fire control group pocket. Someone not knowing any better could buy one of those and upon completion and registration have it confiscated for being a prohibited.