There is another mysterious British children's book I loved as a child. It was called, in Russian, The Adventures of Muffin, and was a collection of short stories about anthropomorphic animals. It also had activities, such as join-the-dots, paper figures, find five errors, and a board game called “Carrots” which was a simplified version of Monopoly and which we loved even when we were quite grown up (we didn't know about Monopoly then). The reason it was called “Carrots” was that the eponymous protagonist was a donkey who loved carrots. Or actually, he was a mule. I just realised it recently: all characters' names were alliterations: Muffin the Mule, Sally the Seal, Oswald the Ostrich, Peter the Puppy, Willy the Worm. In the Russian translation the names did not alliterate, but since we didn't know they were supposed to, we didn't care.
Just like The little ruggamuffin, Muffin's Adventures was not mentioned in any British sources, not even in Margery Fisher's Intent Upon Reading, which at that time was the most comprehensive study. I didn't ask anyone in the UK, because it was obviously one of those books that for inexplicable reasons get famous and popular outside their own culture.In Russia, it was known as one of the most popular children's books in England, and its author, Ann Hogarth, as one of the most important English children's writers.
When I was in Edinburgh many years ago and had some spare time, I visited the Museum of Childhood, and there, among other toys, was Muffin the Mule. He was a puppet and came from a television show. The book was obviously a sidekick.
I asked Morag sometime ago whether she was familiar with Muffin, and she immediately started singing the Muffin tune.
There are dozens of Muffin fan pages and YouTube clips.