The picture above shows what is likely an Iranian-produced copy of the Steyr HS .50 anti-materiel rifle (AMR) in the hands of a fighter from Hamas’ al-Qassam Brigades, in Gaza. The Iranian copy of the HS .50 is known as the ‘AM50’, and appears to incorporate a handful of minor changes. Several such AMRs have been documented in Gaza and, more recently, in Syria. What differentiates this image from others we’ve seen is what appears to be a deliberate attempt to ‘sanitise’ the weapon. It appears that a portion of the right side of the receiver has been deliberately abraded, presumably with the intention of  making the rifle more difficult  to trace. This technique has become increasingly common in recent years, with numerous weapons bearing abraded markings having been documented in Sudan and South Sudan.

Update 02/01/2014: Above left is an image of the Iranian AM50, taken at the 12th International Police, Safety & Security Equipment Exhibition in Tehran, in September of last year. AM50 rifles have recently been documented with a new thermal imaging sight (seen at above right). The source claims an operating time of four hours, in temperatures of -20 to 50 ° C, with a 384 × 288 pixels resolution. Externally, the sight appears similar to Chinese weapons sights. 

Many thanks to Galen Wright for sharing the image with me, and to Yuri Lyamin for information. Image source.