Do we have any reference from WMF that actually defines "Alpaca" as their "base metal"?
Yes, but they're more likely to call it Neusilber or Argentan. I don't believe I said it was the base metal for everything; it was mostly the base for silver-plated flatware. I have old catalogs with front pages that describe the base metals. I'll try to make images of the pages to upload to the forum, but I'm horribly busy, so can't do it now. Silver-plated items often had N (Neusilber), M (messing = brass), or B (Britannia metal) stamped on them, in addition to the marks designating thickness of the silver-plate (I, I/O, 60, 90, etc.).