The pass. plugs look fairly good, but the driver & front plugs look a little dark; the broken plug looks a little dark as well.
There could be a number of problems: rings/walls worn, excessive clearance in the valve stem guides, bad valve seals, or the PCV could also be clogged causing too much pressure build-up which can force oil and vapors past guides/rings into combustion chamber. You're on the right path w/the leakdown test. Get that complete and share the results to better help diagnose the problem/cure.
There could be a number of problems: rings/walls worn, excessive clearance in the valve stem guides, bad valve seals, or the PCV could also be clogged causing too much pressure build-up which can force oil and vapors past guides/rings into combustion chamber. You're on the right path w/the leakdown test. Get that complete and share the results to better help diagnose the problem/cure.
Since it appears to be on one side, it doesn't seem like it would be your rings. I guess we'll know after your leak down test. I'd pull the valve cover on the side that's fouling and check your valve seals/guides. You might get lucky like I did. I replaced all of the valve seals and problem went away, but it is a '67 that sat for a long time.