Housing cleaned up the edge of the flange seems to have corroded away to a chamfer, reaching the stud hole indicated. This can give rise to leaks up the studs, so I opt to replace.
Preparing to refit the head after being converted to unleaded this shows that even though one of the studs isn't exposed to coolant from underneath, it's very close to the recess for the thermostat on top, and in this case the tiny 'land' has been eroded away, and so will be in coolant from above.
Studs with coarse thread at the head end and fine thread at the nut end.
Copy housing in alloy rather than a steel original, but at least it has a good sealing surface.
Not a pretty sight inside. Orange wipes off onto a fingertip, but it's not a sludge, just a film. However if you can't pull the temp gauge bulb out from the head, removing the cover and stat should allow you to push it out using a bit more force.
Thermostat housing gasket showing the notch for the rocker cover. This makes it handed for top and bottom as well as rotational position.
Ready for the new housing ...
... and as usual new parts put everything else to shame. Cork gasket squishes out a bit so don't overtighten, which can also warp alloy housings. However four years later on removal for a head job they were barely more than finger-tight, albeit no leaks, so check them after running a few times.
Pressure gauge Tee fitted to heater hose