All six have one of them. Thanks.

I believe they are (or used to be) grey dots...they signify that they're the updated (stronger) coils...

Does anyone second this?

yes. nissan put a grey dot to easily identify the updated, less likely to go bad coils. the part numbers on the bank closest to the bumper should end in 1 and closest to the firewall should end in 6. or vice versa.