This is apparently a pretty rare bike.  The frame uses the original style Mantra rear suspension before it was replaced with a traditional rear shock design.  A handful of these frames were sent out to be tested and were then called back to the factory.  Somehow this frame ended up in the hands of a Klein sales rep and was never returned.  The serial number leads me to believe that this frame was produced in April of 1995 but we have built it up with parts similar to those shown in the 1996 Klein catalog in which this style frame is shown.   If anyone has more info on these frames, please let us know.

Below is from Darrell Voss who worked with Gary Klein on development of the Mantra:

When we were building the preproduction units, the shock options were not working well for us. We just could not get the performance where we needed it. We had worked with Fox and Rock Shox engineers but nothing was coming from the interaction to support our efforts. 

Internal break through:

After a bit of struggling, we stumbled on putting a joust bumper on the shaft of the rear shock and did some fine tuning of the damper.  This became the first production lots. It was still not perfect, and fell short of our goals, but it was still an improvement from what was out in the market at the time.  You can look back at some of the early releases of the Mantra (first year of production) with the Fox shock on it and see that there was a joust (MCU spring) bumper added to the shaft. This added in creating a nice mid and end of stroke spring compression rates.

 Damper

Not sure if you took the one apart you have but in the bottom there is a damper that uses silicon oil (the oil is compressible). The design of the damper used a fluid logic valve with no moving parts. It changes the resistance based on the impact and speed to the vehicle structure.  The compressibility of the fluid reduces the initial spike on larger impacts while still providing overall control.

 Oilcan shape

We were seeking the best way to enclose the MCU springs and damper unit.  Realizing the cosmetic looks were not as nice looking, compared to most of our work, we where continually seeking a solution to the shock unit. We were under pressure to unveil our suspension efforts to the public. What you have is one of three preproduction units. The other two are owned by myself and Gary and are the larger size (22). Yours is a medium (20).

 Parallel MCU springs

In order to create adequate spring forces, with an MCU, we built two chambers which are stacked on top of one another. In order to keep the structure weight down, we only added material to the outside where the clip rings where positioned on the inside of the chamber for the MCU. The Rock Shox lines of suspension fork lowers have a similar design now (15 years later) with their power bulge in the bushing area.   As for the unified rear frame structure, it provided a nice starting point for Klein to continue the commitment to the mountain market that desired to pedal up and down the mountains. At that, we were focused on evolving the bike structure to provide around 6 travel that could be pedaled everywhere, much like the super-d type riding.

 The challenging characteristic of the bike handling comes in two parts.  First, when coming into tight corners, at high deceleration speeds the wheelbase would shorten up substantially making the bike performance more nimble (akin to a sports car) but, at times, it became too lively for the intermediate level riders. The second issue is more obvious. With the bottom bracket coupled directly to the rear swing arm resulting is more vibrations in the connection of the pedals to the cyclist feet. The Unified rear end made an adequate step in the progression of suspension for XC applications. It would have been interesting to see where some of the next design implementations we had been working on could have taken us.   I guess, we will never know ;)