Bob,
I just remembered why I said 289 -
The same cap was used with the 1967 Cougar Dan Gurney special, which had the 289-2V smooth no name valve covers and twist on chrome oil cap. This particular one came from a 390GT, but I know it is identical to what came with the 289 engine dress up kit on the Cougars. It should not have said Hi Po, that part is a mistake.
Did a 289 Sprint Mustang use the chrome no name valve covers in '67?
That is a twist on cap for a FE. The 67 289 HiPo used a push on cap that is a smaller diameter. The 289 cap is a push on cap also used for the 427 intake oil fill tube. FYI the Autolite marked correct 289 push on cap was used on mid 67 production not 65/66. The Autolite twist on cap for Fe started to be used mid 67 production. Great hard to find part. Best of luck with your auction.