Copyright information was found in the Library of Congress online archive for this issue. The copyright date is listed as March 10th 1944. The cover date is listed as May 1944.
Based on copyright dates, the Grim Reaper's appearance here follows his appearance in FIGHTING YANK #7. He appears next in WONDER COMICS #2.
This story arc is printed out of order, with Part 1 appearing in Wonder Comics #2 and Part 2 in Wonder Comics #4. This is Part 3.