A fetching handlebar moustache, a heavy herringbone tweed suit, pinstripe trousers, a pocket-watch chain, a morning suit, a bright-blue tie pin and a red handkerchief. What are the clues hidden in this new picture showing the upcoming Victorian incarnations of the modern Dr Watson and Mr Holmes? Will they help us decipher what we might expect in their next adventure?

Released ahead of the Sherlock panel at this year’s San Diego Comic-Con – which includes executive producers Steven Moffat and Sue Vertue, as well as star Rupert Graves (or DI Lestrade) – the picture is taken from the upcoming BBC Christmas special, which is reported to involve “a trip back in time to Victorian London”, ahead of three more episodes of a new series.

But how will they get there? Actual time travel feels like it would be a leap too far for a series that has done its best to stay largely within the realms of possibility, and a trippy, 7%-solution-inspired dream sequence might be a bit much for a Christmas episode. How about a one-off re-imagining that steps outside the series’s timeline to show us how this version of the duo would have handled Sir Arthur Conan Doyle’s original cases? (Which, come to think of it, would make it a re-re-imagining …)

Moffat seems to be leaning towards the latter (although he has been known to enjoy the odd bit of misdirection before each season of the programme). As he told Entertainment Weekly ahead of a SXSW panel:

The special is its own thing. We wouldn’t have done the story we’re doing, and the way we’re doing it, if we didn’t have this special. It’s not part of the run of three episodes … It’s Victorian. [Co-creator Mark Gatiss] and me, we wanted to do this, but it had to be a special, it had to be separate entity on its own. It’s kind of in its own little bubble.

Martin Freeman and Benedict Cumberbatch. Photograph: Robert Viglasky/Hartswood Films//PA

How will the mystery of the Victorian identity be solved? Thinking caps at the ready …