Aimpoint Micro Spacer High (39mm)

This is the third category of height hardware and the one most buyers do not know exists. A spacer is not a mount and not a riser: it sandwiches between an Aimpoint Micro base and the Picatinny rail, raising the optical axis to 39mm without changing the optic or the base. Aimpoint sells it as part 12358, with a Low version (12357) for a shallower lift.

39mm is where the height nomenclature actually comes from. It works out to roughly 1.535 inches, and Aimpoint built this spacer specifically to give a Micro lower 1/3 co-witness on an AR-15, which made it the de facto anchor for the term. Reptilia states 39mm, Arisaka calls the same height 1.54 inch, Scalarworks calls 1.57 inch lower 1/3. Nobody is bound to it, though: Badger Ordnance sells a 1.70 inch lower 1/3, roughly 4mm higher.

The spacer only makes sense if you already own an Aimpoint Micro standard or LRP base and want it higher. If you are buying mounting hardware from scratch, a purpose-built mount at the height you want is one interface instead of two, and usually costs less than the base plus this spacer together.