My 76 Touring Paramount has the drilled hubs that you are talking about. I'm not sure the date range, but if the below story is true, it seems like it would have been a fairly limited range as it was a production anomaly.
This was related to me second-hand by Bob Hufford:
"According to the late Dave Staub:
The "drilled" Campy hubs were born at Schwinn. They received a large shipment of hubs that were drilled for tandem spokes, I believe. There were too many for tandem production, so Schwinn redrilled the hubs with correct sized holes in between the original holes which were then enlarged. This gave it an interesting lightweight look. I never heard of any breaking."
-Ben Butler
Brooklyn, NY