Keith,
Given that Ceandess (pronounced C and E) supplied AC with fuel filler caps back in the 50's and 60's, and that Ceandess remains a viable operation on Dixon street in Wolverhampton, still making fuel fill caps, with Director/Sales Manager Peter Killey available at 01902 872000 and on the net at: peter.killey at ceandess.co.uk
Would it be possible to just ring up Mr. Killey, and make a pitch for Ceandess, as the original British designer and supplier, to reissue authentic keyhole fuel caps? Besides the market of AC owners interested in maintaining the authenticity of their increasingly valuable cars, dare we mention the sales potential among the horde of worldwide replica owners? (Consider how many sets of those sand cast aluminum AC embossed clutch and brake pedals have been sold.)
Beyond these arguments for a reissue, an invitation for Mr. Killey to experience some hot laps 'round Donington in a vehicle bearing his product, with attendant PR coverage, might be persuasive too.
Jay, looked at Norton Commando and Triumph bike sites, they don't have the keyhole style replacement cap available, just the ridged square button similar model, made in UK by Ceandess, for a very reasonable $95. See
http://cgi.ebay.com/ebaymotors/Norton-Commando-OEM-Ceandess-Gas-Cap-/310306406189 Art