HP41C - 16 registers missing

[bricator]

Dear all,

We just bought a used hp41c.

Without any module, after SIZE000, in prgm we see "00 reg 47".

We can play with REG/PGM sizes within the 47 regs.

If we add a memory module, It seems nothing change. After SIZE000, in prgm we still see "00 REG 47".

But, after a "hole" of 16 nonexistant registers, we access the memory modules depending or the physical port they are plugged in :
in port 1, we have REG(SIZE + 16) to REG(SIZE + 16 + 64)
in port 2, we have REG(SIZE + 80) to REG(SIZE + 80 + 64)
and so on ...
With REG00 to REG(SIZE - 1) still available.

And we have "nonexistant" if we try SIZE(more than 47), even with memory module(s) plugged in.

What may be the reason ? Memory chip failure ? Address line issue ?
Workaround ? Chip accessible to replace ?

Thank you very much for your help