Gehmann 510 Rear Iris/ Aperature .5 - 3 Mm The GE sight works with most rear sights and adjusts over the wider range 0.5 - 3.0 mm the fixed seat iris works with individually pin-located tempered spring steel leaves aim point precisely maintained throughout the wider range of adjustment by introducing a new system of aperture control operated by varying the pressure of a steel tube against the central aim point - guaranteeing a perfect circle light reflection kept to a minimum by use of matt finishes internally and externally unique Gehmann patent not found in other irises Threading is 9.5mmx1. Not available in 10-32.