That's very helpful LAB - it explains why, when feeling inside the hole in which the steel boss is fitted, all I can feel is a blank surface - it is clearly as it should be, and when the long screw is screwed in, it pushes the larger diameter part in your photo and causes the blade to arc. This forum is great for almost instant advice - thanks.
Very useful info, in case a later bike comes my way. However, long time ago I owned and overhauled a Harris made Bonnie, a 1986 model. I can't recall this set-up on that bike.
Did Harris revert to the earlier system, or is it brain-fade on my part? That wouldn't surprise me to be honest.