I know that others have had this problem, so I thought I would post how I dealt with it.
I just bought this Morgan Chicago 140 4" vise and the Center pivot bolt was missing, I have a Morgan 145 also and it used the same bolt, so all I had to do was replicate.
The shank on the original bolt was almost 1" so I started with a 1" bolt and chucked it up in the lathe and cut it off at the length of the shank on the original. I then drilled it out to 3/4".

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I then did some measurements and figured out the remaining length of 3/4" coarse thread needed and cut a 3/4" bolt with some of the shank and some of the threads. I then inserted the smooth shank portion into the drilled hole in the 1" bolt, centered it up and TIG welded it into place.
I then sanded the head of the 1" bolt down to about 3/8" to match the original thickness. The size of the hex I bigger but it fit just fine on the vise, and works lime a charm.
