Pretty certain the use of the red dragoncon parade flag was a point of contention on one of the colonial marine facebook pages. The facebook group mods apparently were under the impression this was the official flag for use by anyone while at least one AL member was upset they weren't giving credit to the flag's designer (the AL's Rush? I think?).
This was all years ago and I don't remember all the names involved, but suffice to say, I don't think there is an "official flag of the USCM" in canon. You might say this flag below has sort of become the official flag? But might want to seek permission to use and not ruffle feathers. I don't know how many people are still keeping up with this. I am pretty certain none of the parties are active on the board anymore.
This was the flag carried for years in the DragonCon parade (specifically 2014 from this photo. When I returned in 2017 I don't think the flag was in the parade anymore. I may be mistaken, tho).
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parade 5.jpg [ 61.89 KiB | Viewed 17546 times ]
"rudolpht" posted this to the website "your props". No idea of origins or how many may have been made.
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Aliens-USCM-Flag-1.jpg [ 73.11 KiB | Viewed 17546 times ]