Th Gehmann 23mm lens holder for all M9.5X1 Thread Irises allows a prescription lens to be fitted eye-side of your rear sight.   The 501-A is designed for all other manufacturers irises and earlier Gehmann versions that didn't have the ability to remove the front attachment thread.  (For Gehmann irises with the removable front attachment thread see 501).

• Mount your lens (Knobloch 23mm Lenses will fit Or take your lens holder to your Optician to get a custom prescription lens cut) in the lens holder which may be slid off the rail for easier assembly
• Reposition the lens holder on the rail as required and lock it in place
• Positioning of the assembly on your rear sight is easy as the attachment has radial adjustment and is secured with the allen screw located in the front body.