I found these sitting in one of my parts bins and thought they would work great for jacks.
In the milling vise they work great for jaw packing when milling short parts.
The swivel allows for some tilt for odd shaped parts.
They are equipment feet with a 3/8-16 thread so all you need are some various lengths
of 3/8 bolts and you have a large range of clamping choices. I grind the bolt identification from the top of the bolts for a smooth surface.
I found some similar types on the bay:
28 pcs Equipment Leveling Feet W/ Rubber Pads 3/8"-16 with 3/8" Adjustment Range | eBay
Another method for a quick jack is the all thread rod couplings but you don't get the swivel which is ok for some applications.