It is the issue optic that the British Army mounted on the FN-FAL (L1A1). Requires a new receiver cover (mine came with one) and the flip down rear sight. The receiver cover with the mount only fits the inch pattern rifles. A little carefule use of a file and you can make it fit the metric pattern rifles.
It has 4x optics and the reticule is an upside down post. The originals were tritium illuminated, but the ones I've seen sold in this country have been demilled by having the tritium element removed.
I haven't seen them advertise a lot lately but a few months ago they seemed to be available from several sources. Someone was even marketing one as an Isreali version with an AR15/M16 mount. I paid $249.99 for mine from Navy Arms in 1994.