The history of army push bikes for all-terrain use go back many decades before 1979.
But by 1979 they were making bicycle tyres as wide as 54mm, reportedly for Finnish Army use.
So the Finnish army invented the 650B "off road" push bike in 1979 (?)
Any pics of these bikes?
They were used in many countries including Finland. Generally speaking they differed little from mainstream bicycles apart from being strongly and reliably constructed.
From reading about cycling in Finland, I get the impression that their was some kind of tradition of using fat large diameter tyres. This was probably as a consequence of people riding across rough frozen ground in Wintertime. There was also a long tradition if manufacturing specialist aggressively treaded vehicle tyres, for use on snow and ice.
In 1975 these tradditions came together with the introduction of knobbly Hakkapeliitta snow tyres for bicycles.
These tyres were produced for all types of cyclists and in a wide range of sizes. By 1979/80 they had introduced balloon tyre variants including 650b x 54mm and the 700c x 47mm Hakkapeliittas. These probably fitted existing bikes designed for similar sizes of balloon tyres but with road going tread patterns. These may have included army bikes. In the early 1980's Geoff Apps tried to persuade them to make some 700c x 54mm Hakkapeliittas.
But unfortunately they were not interested.