So the "replacement" from that SB has an o-ring that the original didn't - obstensibly to correct for a rough or "off" finish on the surface of the casing. I wonder why they didn't just add a copper crush washer to the old to make a seal? Looks to me that you're only supposed to apply 28 lb ft of torque and the allen is there to keep you from over-tightening - either that or the plug is designed to be "disposable" (single use). Let us know how the new one works out. If you look around there's also a washer that has a neoprene backing that might work to improve the seal against the casing if the one you have doesn't fix the leak.

-- John