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The oil pressure sender (just a switch in this case), and the oil pressure relief are connected directly to the high pressure gallery from the oil pump that feeds the crankshaft. The relief valve is just a spring loaded piston that moves with increasing pressure. If it moves far enough against spring pressure, it begins to open a port that shunts oil to the sump. This drops the pressure in the feed gallery. In this way, the relief valve actually acts as a pressure regulator.
The old switch looked OK, but checked out bad. It should be a short (close to zero ohms) with no pressure on it.
The original switch was a Smiths. I don't know if an exact replacement is available, but I couldn't find one. What some of the Triumph parts places are providing now is the one on the right. One main difference is that while both units have 1/8-inch pipe threads, the original was non-tapered, and used a copper gasket for sealing. With the tapered pipe threads on the new unit, it can't be tightened down on the copper washer without the risk of breaking something. I ran a 1/8" straight pipe die down the threads so it would mount like the old one.
The other difference with the newer switch is that it doesn't have the flange that held the weather boot on. The boot sort of catches on the hex part of the body, but it's not as secure.
The relief valve is dirt simple. missing here are the fiber washers that seal the joints. The spring length and rate determine the pressure regulation point.
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