The links in my previous post will likely be dead at some point, so I’m adding this post for posterity. I found the pigtail described as “C1920, Repair Plug, Single Oval, Hitachi Starters”. This is made by Jimco and their part number is C1920. They make a lot of pigtails for automotive applications. On Ebay it was described as “Alternator Harness Pigtail Hitachi” and sold for $9.50 shipped.