My guess (and it's all guessing) is what the Puch takes is a .833" stem, which is more like 21mm, and commonly used on American bike-boom era 10-speeds (some Ross, some Schwinns, maybe a Huffy or 2, and a few Nishikis, as well as some other Japanese bikes). This is smaller than the standard 22.2mm quill used on 1" steerers, as well as the 22.0mm used on French 1" steerers (which of course are metric, but called 1"). They are difficult to find but not impossible, and you won't have a lot of size selection...try here, try your LBS and then try eBay