The original process is vitreous enamel and they cannot be repaired. Sunbeam Specialties used to stock them, maybe he still does, as i recall they weren't listed in his catalog and I bought 4 for my father's Talbot. These were used on the hubcaps of Sunbeam Talbot's (Mk2a anyway) and on the lower front fenders of the Sunbeam Talbot 90 Mk3. The MK3 version uses the entire part along with the shield. My father has owned Sunbeam Talbots since the late 50's.
I found a set of 4 on ebay UK listing # 143649956877 and sunbeam-alpine.co.uk shows them in their on-line catalog as part #F67