I too think the handle was round, and would have been about 60cm long – held in place at the centre by the two bolts. You set me thinking with your comment about the bolt length, and I’ve been wondering if in fact the bolt length indicates what the diameter of the handle was. I’ve offered up dowel with varying diameters and I think a handle of about 35mm diameter would need bolts that long. (If I put the top domed metal plate with the square holes directly on top of the dished bottom half with the round holes which is attached to the frame, it makes an ellipse, not a circle so that makes me think there was a gap between them, and in fact that allows for a handle/dowel diameter which looks beefy enough to be strong enough to pull the roller.)