The Mk1 has a quite plain stock whereas the Mk2 has a pistol grip cap. The Mk1 has a shorter barrel than 19inches (got a figure in my head of 17 inches- but not 100% sure). I think the Mk2 barrel is quite thick and reduced down under the foresight so as to make use of the same foresight as the Mk1s. My guess is you've got a Mk2.
They were good rifles for the day- pity the trigger wasnt better. I will never part with my Mk1 Vulcan.
Dave