Unknown calendar available only when adding a New Event (Sierra)
This is bit complicated but the only way I know it.

Open ~/Library/Calendars

Leave it open.

Open Calendar > Preferences and set your weird numbers calendar as default.

Now add New event to it.

Go back to the ~/Library/Calendars and look which files (by date/time has changed. That is the one to delete.

Do not forget to change the Default back to your choosing.