@Desmond Griswold can you send a picture of the missing part (presumably a picture from the instructions or pulled from the web)? I have a lantern conversion with broken glass, missing mantles, etc. If I have the part you need I may be able to help you out.
What or “who” was the LP can design? Coleman made them for one, and Hank Roberts branding, and others. I noticed they used a pretty good rubber bead for the nipple. Still supple after decades. I wish some firm would re-issue them. The Picnic stoves were excellent. And the Hank Roberts mini stove was just about perfect for the design. I didn’t have GAZ cartridges becoming obsolete on my radar either. One can safely predict the current flavor of cartridges will therefore probably be discontinued as well. Wankers !