The green plastic Osprey mug has been issued to new recruits for many decades now and is one of those ubiquitous pieces of kit that knocks around and no one thinks anything of. I cannot find an exact date of introduction for the mug, but I suspect it was in the 1970s as a replacement for the china mugs issued up to this point (probably to the great dismay of instructors who enjoyed throwing the china ones onto the ground and smashing them!). The mug was used on bases to drink from and was carried to and from the mess halls by the individual soldier. The mug itself is a simple green plastic mug that can hold ¾ pint of liquid:

It has an angular handle:

And the base has a partial NSN number and a date of manufacture:

These mugs were a standard item to issue new recruits, as recalled by one soldier:
I joined the Regular Army, Infantry, in 1982 so my original issued kit was all ‘Falklands’ period kit – so to speak. We were issued a set of KFS, a green Osprey mug and two tin plates (the tin /aluminium plates were for use at camps where crockery was not provided i.e. central messing in the field or on some camps).
The mugs are also put to many other uses they were not designed for. During firefighting training I found them used to measure out the liquid used to make foam in practice fire extinguishers, a single cup full being placed into an empty extinguisher before it was topped up with water and pressurized.