Lionel?
It sounds like it's tinplate, in any case.
Someone here may know, as it sounds familiar, but it's beeen more than 40 years since I had my hands on a Lionel powerpack.
The tinplate forum elsewhere here on the Kalmbach forums might be better equipped to answer your question if you don't get an answer.
Mike Lehman
Urbana, IL
The folks on the Classic Toy Trains section of these forums (go to Trains.com Sites at the top of this page) are more into Lionel. Folks here may have had them years ago, but are mostly HO and N now. Digging into my memory, I think that the A or B are for one lead, U the other. The A and B have different power. I have one loco that flies, so if folks not familiar with running it were operating it I used the B post. Another loco, much slower, was pretty sluggish on B. I think C was something else (constant power or some such, just don't remember. Good luck, Richard