Backplate:https://www.google.co.uk/search?q=la...bnNceP0AXcwYFg

It's basically a disk, generally made from steel, that acts as an adapter between the spindle and the chuck so that you can use a different size chuck to the one your spindle is machined to accept. On some spindle you need a backplate to fit any chuck.

You should be able to make one with the lathe you have - I did although I cheated a bit and got the router to cut the main profile out since it's quicker. Just a 3" diameter disk with holes to match the 4 bolts on the chuck and 3 bolts to match the holes in the spindle, M4 if I recall correctly. On the chuck side you machine a shoulder to carefully match the chuck and hold in on center and on the other side bore a hole to fit the existing spindle, which again should be a close fit to maintain concentricity.