The empty one was a 'demonstration' photo initially sent to my nephew who's away doing other stuff at present, it's meant to convey the interchangeability of the boxes. Over-winter on a board above a bs nuc; put on a nuc to get further combs drawn in the spring (till we've got the numbers up to our goal figure) and also for filling food combs so that we can get away from using feeders in the mating boxes.
By next winter we'll have some better examples built (as opposed to cut down derelict supers) complete with division boards so that we can transfer two mini nucs to overwinter in the one box.
But yes, the ones overwintering as single boxes are pushing things ...a bit. Although they do have a disadvantage of being shallows but even then our climate should allow them an excellent chance of getting through to spring (as long as we can keep them out of that ever present biting wind).