The on-sale date is the publication date reported in the U. S. Copyright Office filing.
The publication title filed with the U. S. Copyright Office is "Plastic man in the Game of death!" No issue number is included in the filing.
The indicia text:
"This book has been manufactured under wartime conditions in full compliance with all orders and regulations of the War Production Board, in particular 1. 245
VITAL PUBLICATIONS, Inc.
material prepared and supplied
Copyright 1943, Vital Publications, Inc., New York All Rights Reserved
The indicia scan confirms that there is no volume, issue number or frequency present.
Judging from the cover, copyright registration, and the indicia text, it appears this book was originally intended as a stand-alone publication and not part of a series. The Copyright Office treated it as such; the filing is listed in the Pamphlets volume and not the Periodicals one, which is how the Office treats stand-alones. There doesn't appear to have been a copyright filing for the second issue (from 1944), but the cover and indicia point to that being published as a stand-alone as well. This doesn't seem to have become a proper periodical series until the third issue in 1946.
Police Comics 27 is copyright 1943, so its on-sale date was likely in December 1943 and also likely before December 29, 1943, the on-sale date for Plastic Man . Police Comics 28 is copyright 1944, so its on-sale date was likely in January 1944. Thus, it is probable that Plastic Man  appeared on the stands between Police Comics 27 and 28.