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THe fitting must be used as per the mfg instructions. There are/were an instruction/installation sheet in the last few boxes of RT EMT fittings we bought. I can't say fore sure, but I believe the O-ring is to be used. The UL listing of RT fittings was revoked a while back, as the 'old style' fittings were not 'RT'.
A gut answer will be "use the O-ring"
John
