Steve, I do not know of a detent plate for a Mercury. We don't have one for the '64-'66 Tbirds available. The only thing I can suggest is to see if you can find a '60's Mercury Forum like this and ask them about Detent plate availability for that year range and see what they tell you. Here is a link that might help you because I just found some places that sell Mercury parts. Try Rock Auto also, because they give us a 5% discount since they are also a member of our Forum. Let me know if you find anything.
You might try this Forum.