You definetly have a '54 or later full pressure 235 or 261, as identified by the 4 bolt rocker cover. There are a few numbers on the block that will ID what it is, and can be decoded if you can post them. 1 is the engine serial number stamped on the flat machined pad just behind the distributor. This will give the pant code and application, and in some cases, the year(s). The next is the block casting number. This will be on the passenger side of the block, just forward and below the distributor. It will be a 7 digit number, no letters. It should have a large "GM" under it. The last number, which will peg the year, is the casting date. Thsi is also on the passenger side, near where the starter mounts to the bellhousing.
I recall that you've posted about this before, and found a plate on the engine. I beleive that plate may be from a rebuilder, like Jasper.
On the oil filter, these engines, other than the '58-'62 261, did not come with an oil filter as factory equipment. They were almost always a dealer add on, if they did have one. It is a bypass style filter (exept for the mentioned 261), and is not necessary for engine operation. The filter takes its feed from the same port as the pressure gauge, the return is further to the front, not far from the mounting boss for the throttle pivot. The feed line goes in the side of the filer housing, return comes out the bottom.
