Provided by Ed "Sssssssnake" L'Esperance and Mr. Guilt.
U.S. Army --
The MOS codes for Combat Engineers were as follows:
12A10 - Training Combat Engineer
12B20 - Pioneer (basically construction)
12B30 - Demolition
12B40 - NCO
Bridge builders (combat), I'm pretty sure, were 12B60; but don't quote me on that one.
There was a whole other class of engineers that were construction types, and their MOS started with a higher number -- in the 60s, I think.
U.S. Marine Corps --
1300 - Basic Engineer Officer Trainee
1302 - Combat Engineer Officer
1371 - Combat Engineer