Brian,
These are tough. It was common for philatelists at the time to create these covers, then have them favor cancelled or send them to family or friends via feldpost, then have them returned. Many that are on the market today did go though the contemporary postal system, but they were created for a philatelic purpose. But, without the contents of the cover, it's really impossible to tell for sure why they were mailed, only whether they were used contemporaneously (proper cancels, rates, etc.).
For the first two covers, it would really help to see the backs for any arrival marks, censor marks, etc., to tell whether they were actually processed by the mail system or just favor cancelled. I'd check the feldpost numbers to see if they match Flemish Legion units, but I don't have access to my feldpost number literature at the moment.
The first cover 9366006 -- the perfs look like they were cut cleanly. I would expect a little more irregularity if it was a regular joe tearing them from the sheet. The presence of all four stamps in the set on one cover is also indicative of philatelic use.
The second cover 9366022 -- the stamps were cut from the sheets, as evidence by the top and bottom margins. The top and bottom margins have also been trimmed so there's very little white space outside the yellow (compare to the margins of the full sheets in the third lot). That's not normal usage. The presence of all four stamps in the set on one cover is also indicative of philatelic use.
The sheets 9366004 -- these are probably CTO. Way too clean. You might inquire as to whether there's gum on the back, because it would be tough to soak entire sheets off and keep them that pristine. The paper on these is really thin, and very easy to damage or discolor in normal usage. Compare to
these sheets that are still on cover.
Overall, good-looking covers. As I said above, it's almost impossible to divine whether the covers were created by philatelists of the average joe, but I don't see any red flags to make me think they're modern creations.