My opinion: two different types of pre-chamber. As noted, the locking ring on both looks deformed from an improper tool, and you mention a previous owner. If you've got maintenance and repair records, check to see if a pre-chamber was ever replaced or if the head was ever off for work.

If you want to be sure, you need to clean-clean-clean the pre-chamber with the smaller opening. Be careful to avoid contamination into the pre-chamber. Rig a vacuum cleaner to suck out debri, maybe rotate the engine until a valve is open so you can get air flow out of that hole. solvents like brake cleaner or Hoppes #9 may help you, focus on the outer edge to see if you can find a seam at the transition point from vertical to horizontal. I would not go prying around inside that hole until I was darn sure it really is a stuck heat shield.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC