Here are the first two books, each 192 pages with art copied from other editions as seen below. These first two books have covers made of heavy card stock.
Book 122 is The Corpse is Indignant by Douglas Stapleton and Helen A. Carey. It was originally published by New York's Five Start Mysteries, Inc. in 1946 as Five Star Mystery 44, a paperback digest. I can find no information about the authors.
Book 123 is The Waltz of Death by P. B. Maxon [? - ?]. The story started life in the pulp Wonder Stories (May - July 1935) and book form in 1941, published by Mystery House, an imprint of Arcadia House, Inc. of New York. Then Bartholomew House printed the first paperback edition in 1944. A note in the Weldun edition says that Maxon was killed while mountain climbing in Switzerland before the book was published. Confusingly it says the story was originally published in 1946 which is the copyright date on the book. Suggests that the Weldun books were published in 1946.
Weldun 122 back
Weldun 123 back
Five Star Mystery 44
Bartholomew House 9