Sorry, no help here. The ones I've come by have been through others. I've never been fortunate enough to find a75 240 in a wrecking yard.

I'd say follow the wiring from the approximate location of where the distributor was located in the engine compartment to the black box. Maybe someone else snagged it if you didn't find the wiring attached to the distributor when you pulled the engine.

Might also check greenbooks for the 240 to see where it's located

For reference, I just searched around on GCP's site. Shows you the parts, though not where the box is located. You should look at this to see what you still need.

Go to: http://global.gcp.se/

at the bottom of the page, click on the 200 series
click on 240-260-78
click on Section 28
click on 456 (which is the illustration of the electronic ignition components)
good luck!

Section 28; page 456