Expect it to fetch a best-ever dollar figure to match.
The Porsche 904 Carrera GTS has a somewhat tragic history, but it is one which also makes for a very valuable car. The 904 was conceived in 1962 as a mid-engine GT-class race car. It had a fiberglass body, marking the first time Porsche had used plastic composites on any sort of car. Just 100 were built, for homologation purposes, although a large percentage of the supposed street versions ended up being raced by privateer teams, like the GT3s of today.
The engine in early versions of the car was either a 1.6-liter or 2.0-liter four-cylinder unit. Porsche had originally planned on using the new 901 2.0-liter six-cylinder unit, but was unsure of its endurance racing abilities, although a few would find their way into later 904s. More units were planned, but by the mid-Sixties, Ferrari and Shelby had made GT-class wins impossible for cars with such small engines. The car you see here started off with a four-cylinder engine (which the owner still has, and is part of the lot being auctioned off), but has been retrofitted with a period-correct 911-sourced flat-six.
This is not an uncommon practice with the 904, since it's the engine the car was given towards the end anyway.