Steiner had a Ljungman without a mag for sale recently at a price that would be worthwhile for the parts, or just buy his and use your mag and now yours is spare parts. I'm clueless at to what part is the rear block. I had these guns for years and shot one of them a lot, fortunately the only problem I ever had was cracking an extractor after a round wouldn't be extracted and I kept closing the bolt on it.