Rubsign

A rubsign was a small holographic sticker that came pre-applied to Generation 1 toys beginning in 1985, and would reveal an Autobot or Decepticon symbol upon being rubbed. The Mini Spies were the first toys to feature rubsigns, and the purpose of rubsigns was to help consumers distinguish genuine Transformers toys from Tonka Go-Bots and other competitors.

From Teletraan I: The Transformers Wiki, a Wikia wiki.