I had a closer look at this i815 platform

it contains :

FW82815
FW82801BA
SST49LF004A, which seems to be 512KiB flash

Unfortunatelly, it seems that the above flash is soldered to the MBO :-(

Does it mean there's no way I can use the system for L/B or is there some inexpensive way to correct this deficiency?

Not sure if that's a good price, but you can get complete 1 GHZ i815 systems (ie ready for use desktop) here for about $65.

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Thanks!
Shaddam IV