Six/ten meant a 6 wheel ordinary drive truck would get a tag axle with 4 more tires, connected with this belt drive, to result in 10 power driven wheels for maximum traction (thank you Lane G!)

https://www.jalopyjournal.com/forum/threads/vintage-shots-from-days-gone-by-part-2.1154030/page-153#post-13216270

In the early ’50s tag axles were gaining in popularity, but it soon became apparent that they lacked in traction. In an effort to address this problem, a company was started in 1953 in Sioux City, Iowa, producing large pulleys to be installed between the dual tires of the drive axle and tag axle. By connecting these pulleys with a large V-belt, the drive axle powered the tag axle.

Since the tag axle was powered by silent V-belts instead of noisy gears, the company was called Silent Drive.

https://silentdrive.com/our-company/

similar Thornton drive, or Transpec pusher axle

http://forums.justoldtrucks.com/PrintTopic6861.aspx