Sounds like someone run an oil line to head to feed rocker arms. Why, I can't say. Normally oil is fed up through passage in block. Passage may be blocked, so they used an external line. Don't know that putting old filter on would be a good idea. Might not leave enough supply for rockers.
Normally oil filter would tap off of T in block go through filter and return to block. If you wanted to put on oil filter, you should probably find out reason for the line being run to head and fix that first.
