Concept2 Model D Stainless Rail Top Replacement 

They are now well out of production (10 yrs?) and not worth when you can buy a new PM5 for $160.

Generally the stainless topper does not wear out per se. It usually is replaced after somebody drops something heavy on it and dents it creating a speed bump/dip in the rail Which makes for a very annoying seat slide. The only time I replace the toppers is for damage from something falling on top of the rail - usually in a gym