"...this attachment..." Is a stacking hook.
"...paperwork says it is a Mk II..." Look at how the trigger is connected. Trigger on a Mk I hangs off the trigger guard. A Mk II's does not.
No matter what else you should check the headspace before shooting it. You can buy($31.99 each from Brownell's. And you need the whole set) headspace gauges or rent 'em. I would not let my smithy buy them. You will pay for 'em and he will keep 'em.
Then slug the barrel. Not all of 'em are .311".