PeterKerslake says "They don't come much cleaner than this as Cambridge depot's 61671"Royal Sovereign" is readied for a Royal Train working in January 1957 The Royal Family returned from Sandringham to London next day, 24 January 1957, on the Royal Train, at the end of their Christmas/New Year holiday so it was likely this loco was used.

Cambridge depot maintained their pride and joy in fine condition even when rostered to more mundane and normal duties.
The Royal estate at Sandringham, on the line between Kings Lynn and Hunstanton, with its station at Wolferton, was regularly and frequently visited by Royalty travelling by rail via Cambridge and Kings Lynn and whilst Cambridge kept 61671 for Special Royal Duties Kings Lynn depot had its "Royal Claud" in the form of 62614 similarly well kept for use on the line to Wolferton.
The circular panel just visible on the inside valve chest shows where the inside valve head had been prior to 61671 being rebuilt from its three cylinder into a two cylinder "B2" configuration in late 1945, one of ten "B17's" so rebuilt, and it lasted in service until September 1958."