I used to have one of those. Still do - it's in the shed gathering dust.
It's called 'other arms' because it was for use for non-SAS 'arms' of the armed services. That is, just about every other bugger.
They were designed to be a standard pack. And they're good enough. Since there are no compartments, they are positively capacious inside. Mine has the two side pouches as well, that can be joined to make a daysack. With those as well, I reckon I could move house with one.
Good, solid, robust, workmanlike bags. No frills, no disappointments.
Only reason I don't use it is because I try to avoid the military look and because it's so big. That's two reasons.