One of the earliest is Bouquet Knitters Guide, originally published by Dominion Woolens and Worsteds Limited of Toronto in 1949. The art is by "jeg" and is reproduced from the original edition.
Harlequin 71 August 1950
Next is Health, Sex and Birth Control. Published by Toronto's Anchor Press in 1942, the Harlequin edition was the seventh printing of the book. There were at least five further Harlequin printings.
Harlequin 161 March 1952
Third is Why Be a Sucker? published by Toronto's The Metheun Company . The Harlequin edition reproduces the cover from this edition. My copy has a loosely inserted ad which is likely original to the book.
Harlequin 186 August 1952
Harlequin 186 ad
And last is our friend Thomas Kelley recycling his true crime stories with Canada's Greatest Crimes, a paperback original.
Harlequin 435 August 1958 - Art by Norm Eastman