That's interesting that OS didn't mark the piston, I've never seen an OS scneürle piston without the arrow so assumed they all had it.
I'm sure you're fully aware of the requirement for the con rod big end chamfer to face the crank shaft, and that baffled pistons, (pinned) ringed pistons, and pistons with holes and cut outs that match the transfer ports to be fitted the correct way. It's also advisable to keep lapped and ABX Pistons the same way round as they were run in for best compression seal.
Once the the engine is as worn as it sounds like yours is, I guess it doesn't matter as much though. You obviousely have found that Diesels need better comp seal to start than what a glow can get away with.
Davis deliberately makes the comp screw short enough that beginners won't be able to accidently have the piston hit the contra (very hard on the con rod). I replaced the screw on my head to get the comp high enough to run. I found that the contra was protruding well into the chamber. The piston was dark coloured under the bowl and squish band part of the head, but had a ring of clean metal under the contra. The contra must have been very close to the piston.
In theory the contra should be close to flush with the chamber. I replaced the head gasket with a thinner one. The contra is now in a much better position, and the original comp screw now works. I was surprised how much difference a small change in head gasket thickness made.
This wasn't an issue with most original Diesel engines because the contra is full diameter and ran in the bore, they were either dead flat with no bowl shape, or conical shaped to match a conical piston. You could set the contra too close to the piston but the engine wouldn't run that way (unless the compression seal was really low). The conversion heads mostly have the contra run in the head insert so it's possible for the contra to hit the piston, but the engine still be under compressed, if squish clearance is too high. A worn P/L makes this worse.
Dave H