(I give this a try but remember, you're listening to someone who used to time his 235 using a bolt hole in the flywheel in the bell housing hole on the driver's side.)

The later 235's and earlier 261's had a bypass oil filter where only a certain percentage of the oil from the pump would get filtered. Check where the oil pressure gauge is connected to the block. It should be under the manifold about 3/4 back from the front of the engine. My 235 has a tee at that point which also accepted an oil line. The line went to the filter. The second filter line was connected to the block just above the pan in the middle of the engine. The lines are about 3/8" OD.

If you have the oil pressure gauge connected, they must have blocked off the other threaded hole in the block. The engine would be getting oiled to specs or better but the oil wasn't being filtered. Some folks say the bypass system doesn't work all that well so maybe not much was lost if the oil was changed regularly. You can always add the filter.

58-62 261's had full flow oil filters where all the oil from the pump went through the filter using 3/4" OD lines.

By the way, Jim Carter's Technical Article on the 261 doesn't list that number.

1948 3/4-Ton 5-Window Flatbed Chevrolet

34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.