@barbudreadmon Interesting. Following from earlier, I've just switched to mame2003 and it kicked out a .nv which I've opened and I've been able to identify the first three scores 32199, 30000 and 30000.
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The daft thing is, the bytes look very similar to the decompressed bytes I got from the .fs file, if I took half the preceding zeros off. I think I was nearly there. Here's the decompressed bytes from the fbeno .fs file on the left and the .nv file from mame2003 on the right
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Rather enjoying this, but this image keeps popping into my head.
544926ae-fa15-4687-901e-2123f8706994-image.png

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Re: How are .fs files created?

OK, so I've nailed the .nv that mame2003 created, but going to try and nail how the data I've decompressed form the .fs file is structured... so close I can taste it.

b54e7b77-4938-4a07-99bf-d0b55a7d985b-image.png

<game> <name>118 : Crystal Castles</name> <video>/mnt/usbhd/videos/ccastles.mp4</video> <path>/mnt/usbhd/roms/arcade\ccastles.zip</path> <image>/mnt/usbhd/images/arcade\ccastles.png</image> <marquee>~/.emulationstation/themes/carbon-2022/art/systems/arcade.svg</marquee> <releasedate>19700101T001720</releasedate> <favorite>true</favorite> <desc>Crystal Castles (Atari, 1983) Arcade DAZ 32199 FXL 30000 FXL 30000 </desc> </game>