I have a theory about the Donruss Silly Cycles set. The NSU price guide lists the date as 197?. The Sport Americana guides do not list a date.

Hotfoots Theory: These cards came out in 1969 at the same time or shortly after the 1969 Odd Rods set issued in 1970.

Supporting evidence: The Oddest Odd Rods stickers issued in 1970 have fan based art on the back of some of them. All the fan art I have ever seen is Cycle based. If the Silly Cycles set requested fan art be sent in it would have been cycle based. Since Donruss never issued anymore Silly Cycle sets they used the fan art on the backs of the Oddest Odd Rods set.

Just a theory I came up with today after purchasing 49 stickers that included Odd Rods, Odder Odd Rods, Oddest Odd Rods, Fabulous Odd Rods, Odd Rod All Stars and Silly Cycles.