It is actually pretty easy to weld, much the same as Cr-Mo...The big problem with welding Mn-Mo alloys is the risk of the manganese particulates or vapour being released. Nasty stuff: https://en.wikipedia.org/wiki/ManganismYes, you are right, 531 is the approximate ratio. The important thing is that it's Manganese-Molybdenum not Cr-Mo and cannot be welded easily.
It was a super-high tech product when launched in the 1930s. Aircraft wing spars were made of it, notably the Hawker Hurricane. It was also used in racing car chassis in the 1950s.
Good welding practice nowadays is to avoid Mn (along with chromate, cadmium, zinc and thoriated tungstens) without full breathing filtration and skin protection PPE.
All the best,