ECU numbers on GM

[Ron & Cathy Webb]

Recently decided to take a close look at the GM ECU for a
project. Don't know much about them, except they are 68HC11
based, with most of the chips identifiable.

Where can I get info on what the 7 digit ECU numbers mean?
1227747 means what? Is This a hardware mod, or only a firmware
rev? Can someone point me to a compendium of what is known about
the things, so I don't re-invent the wheel?