Hi chaps,
I tried adding the lines to the config file(s) but,as reported, it didn't work with the Steam version but I did manager to enable the marking by the following method.
In game goto the Options menu and then to the General menu. Scroll down to the bottom and click on Historical Decals and change it to yes and apply. Thjis took me to the login screen for the Yuplay website and an option to enable historical decal. I applied this download autoatically. Restarted the game and then went back to the Options -- General menu again and selected yes for the Historical Decals and hey presto they were applied in game. I am in the UK by the way.
Regards
Aces
Edit: Kudos to the devs for providing this option for those of us that like their decals historical

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"IL-2 MAT Manager" program co-developer, Silent Hunter III "Super Turms", "Super Pens", "Crew on Deck" and other mods, Multi-Skin Bombers developer for Wings of Victory BDG version 2.10 + various Steel Fury mods. BoB Development Group (a great bunch of chaps) team member.
Modding is about curiosity, tenacity & lots of patience.
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