keller & sohn
Obviously I asked at Keller's. Went after work today.
Finding it took ten minutes of searching. There's an Antiquariat Keller & Sohn on Kollwitzstraße in Prenzlauer Berg, one of those shops that's been there forever and will survive the oat-latte apocalypse out of sheer stubbornness. Shelves to the ceiling, a ladder on a rail, a cat that is either asleep or dead.
The owner is maybe eighty. I showed him Band I and asked whether he'd ever seen Band II.
"Band II gibt es nicht."
Volume II doesn't exist. He didn't check anything, he barely looked at my copy. Then he asked if I was interested in anything else, in the voice people use when the answer should be no.
On my way out, his son (forties, glasses, friendly) caught up with me at the door and gave me a card. "Mein Vater ist manchmal so," he said. My father is like that sometimes. "Write me. I'll look in the old catalogs."
So according to the man I was told not to ask, Band II doesn't exist. Great start.