Rudy:

The company that makes the screwjacks is Brownell
Boat Stands, 800 533-8433. I suspect they also have a web site.

I understand they "invernted"/brought the things to market.

The screws are 1 7/16" diameter, and come in three lengths, 16" ($11.50), 22 1/2" ($20.50) AND 27 1/2" ($20.50)

My recollection is that they are solid. I will be buying a set this summer.

Hope this does it for you.

Peter