IWS - Individual Weapon Sight
The original first-generation night sight used by the British Army, the SS20 image intensifier was the size of a large Bratty and was of similar weight.
There was nothing 'individual', however, about this kit, as that would imply everyone had one - which they didn't. They were quite hard to come by, and to have one atop one's mighty SLR was a sign of great allyness (probably).
They could also be fitted to other weapons, notably the GPMG and (allegedly) Wombat, though the author of this drivel also remembers seeing them fitted to the old L42A1 sniper rifle and L34A1 SMG - though it's unlikely that the mountings used for these specific weapons were stock items and were probably bespoke made for THEM.
The scope emitted a high-pitched whine upon being turned on. This was quite unnerving as it sounded like a jet engine starting up to the operator. In reality it was quite inaudible and only discernable within a few inches.
The IWS has since been replaced by the CWS and Maxikite. Despite being cumbersome and liable to easy breakage, it was a very good bit of kit and useful for general night observation. It was better than the CWS (apparently).