I have only seen 1 used Slough one on eBay (some years back) with the ‘ribbed’ spokes on 1 edge which I think your looking for as per the image you posted – TOC spares has 1 listed as H35a 7\11\11L 604147 Steering wheel 2 spoke and the 55 Slough parts list in the archive has CIT numbers 604099, 604147 & 604148 but whether Chris still has one (and it is what your after), I don’t know I’m afraid but it’s worth an e mail to spares?
David Faulkner