Are you sure there boards supports the 511000 ones? I'm not formiliar with the board, but support could be a thing. Especially with earlier ones. And to me it would seem that a later board has 4x 30p simm slots. Shop it must be an early one, from the time a 286 was still common.

You could remove the last Panasonics and see what happens. It may as well give you an error in return. Of act l like it's dead until you give back it's ram.

Is it possible the ram needs to be inserted in some other order? For example, u1, u2, u5, u6?

Also there's the possibility there's some kind of jumper setting or BIOS setting to enable the bigger chips?

Last thought: you say 8 chips per row. Is the board ment to work without parity? Could there be some parity chips be positioned elsewhere? If so, they need to be replaced as well.

Notebooks have almost no setting, in this situation everything is fixed, and maybe parity has to change too. Im still testing.

Desolder and solder cost al lot of time, I make it step by step, first remove the rest of the older IC's and than look.
U1 + U2 could be one Bank but U1 + U3 is also possible.
Notebook is from 1991 and Simm expansion is later a standard in notebooks, it has it's own standard build expansion slot for 6 MB RAM, hard to find.

Parity is maby a hint.

When U5 and U7 are removed, then BIOS beeps memory fail, possible U 1 + U2 and U5 + U7 are together one bank.

All 8 DRAM's are replaced 70ns, but still max 640 Kb (or 1024 Kb when shadow is disabled) parity U5 and U10 wrong ?

Anyone knows what's wrong ??