This steel work place has t-slots milled in it for holding down work.  There are tabs on the end and holes in the plate for mounting it.  The item weighs nearly 500 pounds and is about 34 by 30 by 2 inches.

        

The two large inner parallel t-slots have about a 1.625 inch top slot.  The smaller slots look like they are designed for 5/8 inch diameter studs.

  

The images that follow are primarily of the second plate.  AGAIN--THIS IS HIGH BIDDERS CHOICE OF ONE OF 2 UNITS.

   

This work plate is just under 36 by 30 by 2 inches and has a 5" diameter cup machined out of the center with about a 2" diameter hole inside.  Again, this unit weighs just under 500 pounds.

         

There are steel and fiberglass plates that look like they might have been used with the second unit.  They are just under 31" square.  The steel plate is about half inch thick and the fiberglass? plate about 1" thick.  We will include them with the second unit if you want to pay for the extra shipping cost.