This part came with the machine found in a box with all the tools and accessories. I had now idea of its purpose!
So then Stefan in Zurich, Switzerland sent me a photo that explained it all. I have made the missing part that holds the pulley wheels and now the belt tensioner is complete.
I am making new leads screws to replace the old worn ones. The original lead screw is 11 mm in diameter with a pitch of 2 mm. I have substituted this screw with a standard 10 mm trapezoid screw and new bronze nuts. At the top of the photo you will see the original lead screw. Next are the commercially available 10 mm trapeze screw and the 2 bronze nuts. Then at the bottom, I have turned down a part of the screw so that I can put on a tube (heat shrinkage) and turn this tube down to the right diameter of 11 mm.
And here the new lead screw in place with the old one at the bottom. The original nut seen at the bottom of the photo is only partially treaded. This leaves room for the two bronze nuts that have to be inserted to the original square steel nut, and locked in place.