Do you have a picture?
The '208 generator' may refer to a 208V 3-phase generator that may be used for some ground operations, but I doubt they would be involved for start up as a 24V DC electrical system was used on the plane.
As I understand it, US bombers had on-board batteries plus an APU, consisting of a gasoline engine coupled with a 28V DC generator. If ground power was needed, it would be supplied by a 'battery cart' rather than a generator.
Here's an example of (according to the guy in the video at least) a B-29 APU:
https://www.youtube.com/watch?v=M-QV1vC-4kk