this is my notes, Marcel shared this on Discord:
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let me quickly explain...
Copy the 397HUD.mas from your Core\HUD folder to ModDev\HUD
Then open that mas file with the MAS tool and only extract the 397HUD.ini file and also put that in ModDev\HUD
that should be enough to make that HUD show up in dev mode
if you edit it, the ini file can be saved with the updates
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if you need to change textures, you need to change those by adding them to that mas file
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A while ago it worked for me and I was able to modify S397 HUD to my liking. HTH