It was a big tank where you dumped in calcium carbide chips and sealed the lid. Then water would drip onto the carbide until the pressure came up and the drip stopped - Run the lights or the torch and the pressure drops, and the drip starts again.
Every few days or months (depending on size and use) you'd shovel out the hydrated calcium goop, and refill it with fresh chips - which is why you had two (or more) generators, so when the first one pooped out you could switch over and service the exhausted one.
For welding shop flows, you might need two or three generators to keep up.