O ring may have been round, being installed for 24+ yrs may have made it look square. I know I couldn't get a round one in, but once I squared one or three off, I got it to fit snug.
When this came up before I thought I saved my old leaking heater, but when I looked for it evidently I'd trashed it. And my O-rings weren't leaking, I could have measured them and maybe ID'd them.
Heavy truck and diesel mechanic (thankfully retired after 30+ yrs)
'91 F-250 SC 4X4, 5spd. ATS Turbo, 3:55 diff.