the binoculars have grease in their ocular lens assembly to permit focussing, and in their hinge asembly.
In extreme cold weather the mechanism might freeze solid unless a suitable grease was used. Various greases were progressively introduced to cope with the conditions that ultimately ranged from-40C to 50C.
" KF" intro May 1940 -41 servicable to -20 C
"O" Aug 42 -40C
"+" Nov 42-44 -40C
"blue Triangle" late 43 - 45 -40C to 50C