My '51 is the same as Mark's. Nickel I believe and a painted end cap. I have a spare dynamo and it is also nickel plated but not as nice so I will probably re-plate some time in the future.
'51 SQ4, '37 VH500 Red Hunter, '56 SQ4 (basket case), '49 Sq4 (basket case), '48 A7 BSA Twin, too many other BSA's, Norton's, Triumph's, Ducati's and Japanese to list. Just crazed is all my problem is.