Here is an Ebay auction for an NOS C3OZ 9A589 C Hipo 289 carburetor spacer plate by Ebay member "yourinternetauctionstore" with a Buy-It-Now of $125.00:
NOS Original Ford 1965-1969 Mustang 289 Hi-Po 302 351W 4V Carburetor Spacer
Original new old stock Ford part without box. Factory carb spacer. I do not see any bolt marks so that is why I believe it is NOS. 1" tall.
NOS Original Ford 1965-1969 Mustang 289 Hi-Po 302 351W 4V Carburetor Spacer MFRB | eBay
-Fred-