I've seen conflicting information for monster.md, so I've lost interest in it. I may revisit it when I have more time.
In the mean time, I might make an editor for item_p.bin (as described in this spreadsheet).
Quickly looking at the SIR0 format, it seems to be 4 bytes of header, 4 bytes of a start index (maybe), and 4 bytes of the end offset. The next 4 bytes are usually if not always all 0. The data at the end I'd guess it garbage, but after removing it from various BALANCE files (ensuring all removed data was after the apparent end index), this happened when starting a new game (attached). Also I picked up money (the strange icon on the floor) and see it in my inventory, and got the message about how it was the world's currency. I could throw it too. Then the game crashed when using a regular attack. The data at the end appears to be garbage but is apparently significant. Maybe it helps when spacing files in the ROM's file system or something.