It's easy to separate the boards on an O59 DME...

Follow along with me now...

Once the DME is out of the car, and the metal cover has been removed by
bending the tabs...

Look on the dead center of the metal bottom of the DME, and if there is a
PLASTIC sort of rivet thing there.. pull the center of it out and remove
it..

Not ALL of them have this rivet, so don't worry if there is nothing in the
center of your DME bottom

Now turn it back "board side" up..

Notice how the "back" end of the boards are joined with a ribbon cable
setup..

Looking just to the right and left of the cables you will see a set of
plastic risers that will pop apart the "left" one usually pops at the top,
and the "right" one at the bottom.. look closely then pop them apart

Now, with the back of the boards free.. pull them as far apart as possible..
now pull the top board back, and the front of it will come clear of the
connector and you can "unfold" the whole assembly...

The two boards will be side by side, connected by the ribbon cable
assembly..

Chip is on the board that was the "top" board and is the ONLT removeable
chip there..

It's either a 24 pin chip numbered 1 267 355 169 or 1 267 355 206.. or a 28
pin numbered 1 267 355 353