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[Eagle of Fire]

Please disregard as I've found the reason myself.

Digging further, I've found this page about ram on KM4M motherboards:

RAM

*

Both the L and V motherboards have two slots for 184-pin DDR DIMMs. Both can support up to 2 GB of PC-1600, PC-2100, or PC-2700 DDR SDRAM.

I've found a PDF with exact specifications of the salvaged memory stick I've got and it is listed as a "240-pin DIMM".

It is quite obvious that the reason why it is listed as incompatible is because of the difference in pins, even though both sticks seem to be exactly the same size.

Oh well. One way or another it was free to try.