I'm no authority on postal markings, but I have viewed thousands of WWII patriotic covers on
ebay over the years. I do not recall ever seeing this particular imprint. I doubt that there would be any official markings for free soldier/sailor mail since all that was required was that it be marked "free" and the sender include their service number along with name. This was likely created by sender, or someone they acquired it from, to provide a more distinctive "free" marking.
Note that this cover is cacheted, so it probably is philatelic in origin, which would go with the special "free" marking. Normal soldier/sailor mail would not have a cachet, and in most all cases just as a manuscript "free" where the stamp would normally appear.