Topps launched a very high profile collectors club in 1975, although it is not too well known today. Dubbed the Topps Sports Club, a coordinated effort across all four sports brands was launched, with wrapper side panels ads enticing kids to join. The ads were similar on each sports' wrapper:
The hockey panel is from the well-illustrated Vintage Hockey Collector Price Guide. The panel looks like it was identical on all four wrappers and it also came on the Mini Baseball pack as well. $2.50 would get you in:
For the record, the mailing address was a loud shout from my childhood home. There was also at least one print ad, probably in Boy's Life magazine:
That is from the Project Baseball 1976 site by the way. The newsletter that came with membership started off at 6 pages and then was winnowed down to four. I only have the second issue, which features 1975 Football:
As a bonus you got an 8" x 10" color photo of a popular superstar tucked away inside:
Griese is printed on very glossy, thin paper stock. Other photos in the set seem to include Joe Morgan and Steve Garvey, Bobby Clarke and (possibly) Dave DeBusschere. There could be more. The Garvey and a little more backstory can be found here.
The project seems to have died out pretty quickly; I suspect the price was a little too steep for the times.