The way you described the male threads on the filter and with female threaded ring sounds like my pool (unless I misunderstood your descripton).

The threaded ring or collar you describe is straight thread (not pipe thread). It is the same as a standard pipe union for 2" pipe (at least in my case).

You can buy one at http://www.bestbuypoolsupply.com/plumbing-supplies.htm an djust use the threaded ring/collar.

If I'm way off on understanding your situation try to post a picture.
Good Luck