It's a GC, but as above it will be so interesting to see if the residence since date coincides with one of your visits (hopefully) or with the phone call you had with the agent (hopefully not). Either way I'd definitely order your full immigration file via a FOIA as @OldUser suggested above. If an error is found in the future, even after citizenship, you could have your status revoked, so best to make sure it's legit.