It seems likely that the 868 mis-number is not a mis-number at all, but that 868 was in fact Stuart's telephone number. There are one or two points in favour of this theory.

1. The 868 cards all seem to be relatively late (post 1915 or so), and of lower quality British manufacture.
2. The ones I have seen seem to be reprints of earlier cards.
3. Southampton 868 sounds like a phone number.

Unfortunately I have been unable to prove this to be the case one way or the other. Southampton Library has a collection of telephone directories, but they do not seem to go back quite far enough and none list Stuart.

Thanks to Edward for the updated information.

Some examples of 868 can be found here.