The straight one looks like a typical manual steering Mustang V8 center link to me. I have a number of them salvaged from 65/66 parted out Mustangs.
My guess is that there were either 2 suppliers or a change over during production. Out of curiosity I’ve been looking at several suspension pictures online including this one that Charles was involved with.
http://www.saacforum.com/index.php?topic=12072.0 In image 44, it’s very clear that the center link end has the dimple on the bottom and there are many more examples.
Besides that and the tapered end, the cotter pin holes are different sizes 9/64 for the dimpled version and 7/64 for non-dimple. The non threaded part of the stud on the dimpled version is about 3/4” and the non dimpled is around 9/16. Pitman arm is 5/8” thick. This may be a hint at a redesign because even though it was snug, mine came out easily without needing the normal tools to remove a tie rod which make me think the castle nut was bottomed out
Also included 2 pictures of a “NOS” center link currently on Ebay