Its been documented elsewhere that when the class 21 > 29 conversions were done, the finished locos didn't all retain their original numbers. I've seen a number of photo threads where people have tried to work out why a converted machine has the number of one that wasn't converted.........I think there's still a lot of info out there to discover.Another question is whilst the cab carries the number D6122, weathering underneath revealed D6121. So what is the real number? D6121 had" eyebrows" above the cab D6122 didn't . Were the cabs replaced or did the locos switch numbers?
This photo shows a 21: no headcode boxes and it still has nose doors
the list of conversions according to Wiki:
D6100–03, D6106-D6108, D6112–D6114, D6116, D6119, D6121, D6123, D6124, D6129, D6130, D6133, D6134 and D6137
This photo is of 6122 which matches its unconverted status, 6121 was a 29. However I'd bet the numbers were switched.