I have an exact same vise, the pin is a tapered pin to hold down the swivel jaw to be perpendicular to the dynamic jaw.
mine was stuck so I cut a slot with a cut-off wheel, used a big screw driver to turn it lose. an impact driver would be ideal tool but I don't have one.
That, my friend is a beauty and the paint looks to be original. Keep is safe, use it, don't abuse it, and most of all respect it. it will become the second most important thing in your shop besides you!
