Maureen,
actually we know that they did not remove any smurfs all the way to 1978, just added new ones. I don´t think they made any changes the last year they made smurfs in 1979.
I am 100% sure the picture of the smurfs here is taken during 1978. So these are the smurfs Bully sold 1978.
I have the very large Bullyposter from 1978 without smurfs and if you compare they even added a few more items they did not had before.
So in 1978 Bully sold all 36 smurfs they had made that faar. And Bullyvariation of Gargamel, Johan and Pewitt.
I forgot to add that I am also pretty sure Bullyvariation of digger, Gargamel, Astro and Smurfette were never sold by Schleich.
The only thing they changed are that they sold and made the Schleichsmurfs(-molds) between 1975 and until 1977. When Schleich got these back and therefore they were not on the Bullypicture from 1978. This is also why they as said before added their variation of Astro and Smurfette after the new smurfs to be sold in 1978.