Pinned vs non Pinned - was the pin to help keep the bolt from flying open at inopertune times. Back around 67 or 68 I bought a gun digest that had a pretty long discussion of the Ross Rifles, wish I still had it.
The infamous - although in fact extremely rare - "blow back" problem was only possible with a Mark III - aka Model 1910 - action, but could occur if the bolt assembly was reinserted into the receiver (after cleaning, for example) with the bolt-head improperly positioned - so that it then would not rotate to engage the locking lugs when fully closed.
However, to quote one of my references on the Ross rifle:
".... improperly positioning a bolt in a 1910 action isn't the easiest thing in the world to do. Actually, incorrectly assembled bolts-in-sleeves slide with difficulty - very often with great difficulty, if at all -into their receivers. This, in itself,is enough to warn any discerning rifleman that all is not well."Quoting from the same reference in answer to your question:
"In 1915, Canadian armorers in England worked out a system of foolproofing the Mark III bolt assembly against incorrect positioning. This simple alteration, carried out on all Canadian-issue Mark III's, consisted of putting a rivet through the sleeve which followed a recess cut by filing down three threads of one of the bolt spirals." [This refers to the spirals in the bolt body, covered by the sleeve, which produce the rotational motion when the straight-pull action is worked.]
Finally, even with an "unpinned" Mark III Ross, there is a very simple visual test for ensuring that the bolt-head is properly positioned - the locking lugs will be positioned about 1" (i.e. approximately a thumb's width) out from the bolt sleeve:
With the locking lugs of the bolt-head properly engaged, the Ross is actually a very strong action - withstanding pressures approaching 150,000 psi!