Alfred Mainzer was the postcard publisher not the artist, according to this informative article in the Mousebreath Magazine The artist was Eugen Hartung from Switzerland. I have some unused postcards and I see that they sell on Etsy for up to $15 for a single card. I also have some written on by my father who was a definite fan of these cat cards. I know he used to buy them in the ’50s from a card shop in Montclair New Jersey run by Mr. Bert DeCamp.
The artist signed his initials in the corner in a heart which I never noticed before but I read that on Mousebreath
The Hartung postcards were first published in Switzerland by Max Kunzli of Zurich and then(from the 1940’s onwards) by the Alfred Mainzer Company of Long Island New York. Each card is signed with the artist’s monogram, a little heart in the lower right corner. (Because of cropping, sometimes the heart gets chopped off.)
Some are printed in Turkey and some printed in Spain.
LOOK WHO IS KNITTING A SOCK (using double point needles)