In reply to Lizard Ollie:
Hi Ollie,
Thanks for excellent reply, great man.
I've checked spots you mentioned. Including bit 'on otherside' I hadn't spotted where the cam rotates and rubs on the underside. I think that's what you meant, either way, good tip, will keep an eye on that.
Its a 2016 model, does indeed have a machined groove for the slack rope.
For anyone's curiosity, and my mutual reassurance, whilst the device is missing lots of colour (and judging by the muc I cleaned out it has been dragged through some dark/damp places) there's no wear identifiable at the touch, nor anything worryingly sharp or perilously thin. So I'd be happy to use it for light duties backed up with ascender/fall arrest/clove hitch as I intended.
HOWEVER, whilst I paid less than half RRP I suspect it has much less than half its reasonable life left. Moral of story; used gear can be a false economy!
Think I'll stick with open-box/ unused stuff on eBay in future!