From what I have found the only factory rear air offered during the 60's and 70's was from the factory and then they were usually found only in the 600.

You can tell a factory installation from an aftermarket by the air inlets for the rear unit. The Factory units had air intakes on the outside of the car; small air scoops mounted on the sides of the car just behind the rear doors.

There were also large scoops you could get if the factory thought you needed them.

The after market units were box shipped units that mounted in the trunk in the space under the package tray and between the real wheels. This must have made getting to the bolt that holds the rear axle support to the car quite a chore.

These units were heavy, usually about 45 pounds, and their air intake was in the middle of the package tray where a 6x9 speaker could be mounted. The air outlets were to either side of the package tray where 3.5 inch speakers could be mounted. The outlets had grills that covered them to keep you from dropping things in there and air vents that directed the air away from the back glass and into the interior.

The controls for the unit were mounted under the dash where the driver could reach them.

The whole thing was secured by pieces of usually angle iron that had to be cut to fit. Supports under the units helped to keep it off the floor and up against the the package tray.

The whole thing was tied into the front under the dash AC. The front AC was to keep the front passengers cool and the rear was to keep the entire car cool. The theory was you would have the car's interior cool without air blowing on your face or hands. Older people are known to dislike the cold air blowing on them so this was the solution.

There were normally two blower motors with easy to obtain motors. This way the owner could be sure of finding a replacement blower motor if they were away from home.

These come up on Ebay now and then, and I don't know of any additional value a factory unit would have over an aftermarket unit. With a factory unit you would need to cut holes in the side of the car for the inlets since there is no central intake through the center speaker hole.

By the way, this takes up so much room in the trunk that there is no way to install rear speakers unless they are mounted above the package tray.

This installation also requires a drain hole be drilled in the floor for the drain hose to go through. This is no big deal but it is something you would want to do before mounting the unit. Space between the wheels is tight after the unit is installed.

One last thing.... These aftermarket units were built to fit a large number of cars and none were built to be Mercedes specific. If you start looking for one then I would recommend an ARA as they seemed to build one of the strongest units for this type of application.