Just a heads up, I bought 2 of the 3 gallon pin locks mentioned earlier in this thread (I use them to lager in a dorm fridge). Got them last week. They are identical to the 3 gallon ball locks I have, size and everything. You could change out the posts to make them ball locks, the spartan/cornelius posts are the same, or at least they seem to be on mine. Although the lids have a pressure relief valve, they're not the kind with the ring you can pull to relieve pressure, you still have to hit the gas post to relieve pressure on these.
Be prepared to clean them, nasty is an understatement. I almost wish I had gotten the new ones, not sure if it's worth saving the $20.