I've never seen a stovebolt charging system with a type B circuit, and I've been working on them since the late 1950's. The simple way to tell the difference is to look at the brush circuit at the back of the generator. If the field coil wire is attached to the armature output brush (the insulated one that goes to the ARM terminal of the generator) the generator is for an A circuit and it gets its ground at the regulator. If the end of the field circuit is attached to the grounded brush, or the generator housing, the generator is for a B circuit. The internal wiring of a B circuit regulator is also very different.
Jerry

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