The large bolt in the recess on the head just behind the timing chain area and close to the fuel filter mount is part of the drive mechanism for the oil pump. An oil leak there has nothing to do with the head gasket.

Forgot to mention, compression readings will also be dependent on the speed the starter motor turns the engine. Suggest you use as fresh a battery as you can, hook the battery to a high output battery charger/starter, let the starter cool about 2 minutes after checking each cylinder, open the throttle all the way, crank the engine at least 8 revolutions per cylinder to get a maximum reading that is consistent for comparison, remove either all the glow plugs OR the injector nozzles depending on which compression gauge fitting you're using, shut off the fuel flow to the injection pump (either clamp the inlet fuel line or use the injection pump's manual shut off that requires disconnecting the throttle linkage).

Compression can be low if the valve clearance is too small. Usually, M-B diesel engine valves lose clearance between valve adjustments. If compression seems low, you may want to check the valve clearances on the cylinders with low compression that otherwise may be a good used engine even though the compression is low due to valve clearance being wrong.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.