Among the smurfs I had for sale on eBay was a Bobsled super smurf that no one bid on during the two times he was offered for sale. To help him deal with his low self-esteem issues, I looked at him carefully with a magnifying glass to try and find something to add to my listing to boost bids. There, on his "unwanted" body was a Muster Mark!!! When I mentioned this in a forum, other members started writing and asking me about muster marks. I am no expert, but I do have my old SSCI newsletters - one of which gave the following explanation of these marks:
The explanation given to the Smurf Collectors Club International by the Schleich factory about these M marks was that "We have a man in the company who controls the figurines we produce. He always picks a few figurines among the new ones which have been injected and, if they are okay, he "Brands" the figures with this mark."
In other words, first run sample figurines found satisfactory were hand-branded with the special M mark that stands for muster (which means "Sample" in German). These were done in very small quantities. All of the figurines with this mark are West German variations.