x000000 47 52 49 4d 04 00 00 00 85 c5 0e 00 78 9c ec bd 'GRIM........x...'
If I add a single map note, the save file gets almost 2KB bigger, and the header changes to:
x000000 47 52 49 4d 04 00 00 00 81 c2 0e 00 78 9c ec 7d 'GRIM........x...'
I'm assuming that the 04 is a version tag or something similar. None of these look like length values for the save, and it's not immediately obviously IFF or zlib (the two things I look for first). In any event, I see no zlib stuff. I do note the game's got embedded Lua in it, which suggests that you could have a ton of fun if you could find a script console.
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