no, not a 550 series, but... 25 Plus. it was a kind of teaser 😁
don't look for it on google, you'll find nothing at all. this is what we can call a kind of "VINTAGE COLLECTOR RARE" machine, priceless...

so, it's a "semi AT" clone, as it's a 286 @ 12,5 Mhz, but with only 8-bit isa slot. but got an onboard ide xt interface, and vga. it's really a compromise machine...
i had the chance to get it with original manuals, dos floppy, keyboard, and his box. and it's a nice looking little pc, from my personal taste.

but of course, i got an issue with it. it doesn't want to see any extended ram.
this machine use a mix of zip chip and 2 regular 30 pin socket, for acheving 4MB.
so, i got a set of zip chip... first try was wrong, i used 256kbit x4, and he doesn't support them, so got 1mbit x1 and this time, it's ok.
the onboard 1MB are 256x4 zip chip, by the way... that's why i tried that first.

when you add extended ram, the onboard ram is disabled.
so, at boot it scan for the 640 + 384 reserved ram, till here, it's ok... he see the external chips and use them for the base ram, but he didn't scan beyond 640KB...
what can be wrong in this case ? it's a bit like if the bios don't have the ability to go beyond.

i checked the zip and simm pinout, to see if it's ok, and it is.
the pcb silkscreen give the simm and zip references : MB85230 for simm module, which use standard pinout, and 811000 for the zip.
811000 give nothing on google, but i guess it's 81C1000, and you find the schematics for them. and they are standard pinout zip.

all is configured by switches, all is set according to the manual.
so, for the moment, i give up. i don't understand what happen here. if you have some ideas...
thanls 😀