Small extra SSE board

[MacTopus29]

While restoring my mac 128k I took a look at the logic board and found an extra sse board. (pictures below)
Does anyone know why it is there, what it does, and if it is rare or not?
Thanks

 

[mg.man]

It looks like your "128k" has been after-market "upgraded" to 512k - pretty common since there wasn't a lot you could do with a stock 128k. It looks like the original "64k" RAM chips have been replaced with "256k" (now socketed) chips.

That extra 'board is probably for decoding the additional address lines needed.

 

[Phipli]

It looks like your "128k" has been after-market "upgraded" to 512k - pretty common since there wasn't a lot you could do with a stock 128k. It looks like the original "64k" RAM chips have been replaced with "256k" (now socketed) chips.
View attachment 47460
That extra 'board is probably for decoding the additional address lines needed.
View attachment 47461

I thought you didn't need extra decoding for 512k - isn't the story that they routed the extra address lines to the memory without telling Jobs?

 

[mg.man]

I thought you didn't need extra decoding for 512k

IDK... maybe someone here does? Both my "128k"s have been upgraded - one had a board swap (to 512k) the other an aftermarket u/g to 2Mb! That upgrade was pretty extensive to say the least!