Best I can tell, Lucas calls them (wait for it)...covers! Probably the only way to get them is to scavenge off a parts car.
EDIT: Oh wait; I think I've misunderstood what you're looking for? It's actually a panel on the body, up under the dash area on a plenum? My TR4 SPC calls them "Plate, cover, windscreen wiper cable access" -- p/n 650159. Screws are YA0403, which are self-tapping pan head #8 screws, 3/8" long. From TRF's online catalogue, it appears that the parts are same for TR6; unfortunately, they're listed as NLS. So back to my original advice: parts car scavenging! :laugh: