I cannot comment on the Harrison 03 book, but his Garandbook is close to a disaster. He has lots of errors, but having researched Grenade Launchers, one instantly comes to mind.
He has a gas cylinder lock screw listed as prototype, which was a fantasy item which was never produced or existed.
I have the development history and Remington Patent information of that item and my guess is his book stated that to justify one on his rifle.