Here is something I found out about a similar model that was produced, and looked it up after I saw one for sale at a gun show.
"The S&W 4596 was a one time production run for Lew Horton. It is a .45 ACP built on the 4506-1 frame, and fitted with a 4516-1 slide; with Novak low mount fixed sights, and 1st triggerguard. The book says that an approximate total of 382 units were produced in 1990, with a 3.75" barrel. Product code is: 104490"
I do not know about the 4566 frame comparision.