The port with a dowel in it (red arrow on your pic attached) is the vacuum port in the intake manifold. That would be routed to a vacuum wiper motor (assuming you have one), not an oil filter. The port indicated by the yellow arrow is the pressure port for the gauge in the instrument panel and a filter if installed. The port indicated by the green arrow appears to be the return port for an oil filter, but plugged with a recessed plug that someone tried to remove (unsuccessfully) with pliers or a pipe wrench.
But a filter connected to those two oil ports would be a bypass filter, just filtering a portion of the oil being sent to the engine. So running without a filter wouldn't kill the engine. There are lots of engines that are running around without filters. You just have to change the oil on a regular basis to drain the crud out.
Welcome to the Stovebolt forums. You should go over to the Welcome Centre and tell us a bit about yourself and your truck (how you came to own it, etc.) Post pics of the truck as well.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
