These are postcards from Mutts by Patrick McDonnell. Like many other cards and stationery based upon comics or graphic characters these are published by Chronicle Books, however I purchased these from Amazon.
This is the front of four seperate cards each of which was the title panel of a Sunday publication, the backs are simple with lines for the addressee, an image of Earl the dog sitting and Mooch the cat sleepwalking. It also provides the date the comic was originally published.
These cards are extremely fountain pen friendly and accept the ink withour feathering or smearing.