I recall Andre mentioning this before but I wanted to expand on this. There are a small number of smurfs that instead of being given a mustard paint dot it was given a mustard paint circle. I believe this was an intential! From my understanding these were given to smurfs that were sold in the UK in 1978/79.
I have two smurfs with this mustard paint circle - Papa Smurf and Shiver with yellow scarf. Both of these smurfs also share the same markings - W.Germany © Peyo
Can anyone else add to this list?