Hi Stephen,
It's more than just a spacer, if equipped with the missing threaded piece, tightening that oiece by hand clamps the wheel in position so it won't turn, loosen it again and the wheel turns - it's a wheel brake. If you get in touch with Cliff (not me!) he may well pit you onto a source for the correct parts.
Absence of the brake should not cause play in the steering, that's more likely to do with poorly adjusted cables. Adjustment is at the quadrant, but don't set them too tight as that causes excessive wear in the conduits.
Peter.