Books 100 to 104 have similar design features on the front cover and spine. Book 105 doesn't exist and 106 has important differences. The book number appears to be handwritten, the text feature at bottom right is gone and the spine has differences. In fact book 106 has features identical to those of another publisher, Alval Publishers of Canada. Compare Travellers 106 with Alval's Crow Pocket Edition 27 Published in July 1949. Alval is Alexander Valentine of Toronto - more on him later.
The artist is the same and the author name is written in the same hand. Even the title is a "Crow" type title.
So what does any of this mean. One interpretation is that the first five Travellers were published by persons unknown (Canadian?) and Valentine tried to continue the series, mistakenly thinking that there had been a number 105. What the books stated publisher, Ward-Hill of New York, has to do with any of this a question that likely will never be answered.