California Pony Cars is reporting the elusive cap is in stock.
https://calponycars.com/collections/1964-1973-mustang/products/1966-gt-350-horn-button-center-cap-for-wood-steering-wheel (https://calponycars.com/collections/1964-1973-mustang/products/1966-gt-350-horn-button-center-cap-for-wood-steering-wheel)
CJ Pony Parts and NPD is out as well.
BUT, one on Ebay:
https://www.ebay.com/itm/127108418607?_skw=1966+Shelby+GT+350+Steering+Wheet+Hub&itmmeta=01K9ZJ2M27BQPMXBSYW498KQGE&hash=item1d98408c2f:g:ihcAAeSwEw5oIosz&itmprp=enc%3AAQAKAAABAFkggFvd1GGDu0w3yXCmi1cJpbYt7n%2FnDiBEDcWtyGmz4lCIujuc4F4vw5kazgfCbNdGnbdPMmDFdWe4S8MaT9aYSKxh9ik4q%2Fgx2kxg3LoihRRurmks2T%2F5jCSIgCzrKEQXdOQANNuXhqSDlCVHC9ucR%2Fc3tuV2zhlGZ0oHcFsoWTaor8GvGXE7md0PajBAH6GlLUe2HjBM0iIHMe%2Bq6Be1IvEJ2hkSVbiXBXdr8GNcn7rx%2B2lLurdAjsa0S41k5oDGCX6pqe3quXzzgRINIOxnejomn6AEC8giWHYRcD5FPryMoCQyDqnqfKWbjMNf0MIywcb%2B8XmoC1JLodwd1t4%3D%7Ctkp%3ABk9SR6TCivLPZg
Was on their "notification" list. Received an "action alert" from them today with the ability to order online. Their discrepancy, not mine. FYI.