AFAIK they are not available in "chrome" but John Worrall at Heart of England Morgan has them in stainless steel

see:
http://www.heartofenglandmorgans.co.uk

phone 01299 250141

look under lamps, lights and related fittings

REAR LIGHT PLINTH STAINLESS STEEL
Reference: #ELE 116
Replaces plastic item with Highly Polished stainless steel.
Retail Price: £45.00

Hope that helps