For your 2021 Keystone Montana High Country 331RL rear roller blackout shade, the key measurement is the fabric width of 71.75 inches and a height of about 36 inches. The shade you described has a black side facing inside and a white side facing the window, typical of our Value Line and Slow Rise roller blackout shades. To replace it accurately, select either the Value Line or Slow Rise model with these dimensions and the black inner/white outer fabric configuration. Mounting is via a snap-in roller bracket above the window, which is compatible with these models. If you need a precise fit, confirm the exact height measurement and mounting style, then order a custom or extra-wide shade with these specifications from Elkhart RV Parts for a perfect match.