Lenght is 2,8 cm or 1,102 inch
 Diameter 11mm or 0,433 inch
 No thread to keep it in place,no markings
greetz,brummbar

Unidentified3

Unidentified2

https://www.warhistoryonline.com/war-articles/look-views-down-the-sights-of-historical-military-firearms-very-clever.html

A great project and work from the guys at www.candrsenal.com. They have spent a massive amount of time taking hi res images of historic firearms. Started their Facebook page this month and can find it here CandRsenal Facebook and they are also on Twitter. For all the gun fans and collectors out there and by the looks of it, it is going to turn into a great reference site for firearms.