It might be worth a look around t’other side of the engine. I haven’t found a picture with an oil pressure switch on a 400 or 600 near the oil filter so far. Although this is a 600 engine the 350/375 and 600 were all designed around the same time, the architecture is similar but not identical. There is definitely an oil gallery on the o/s side of the 600 since some versions have the water pump on that side and it requires an oil feed. I have had to make a new pipe for it in the past. It should be easy enough to see in a TK although it might be behind the gear linkage.