TOLKIEN (J.R.R), THE LORD OF THE RINGS, de-luxe edition, 3rd impression,, pictorial black cover and two section box with paper label, George Allen and Unwin, 1974
ALDIN (C), THE POSTHUMOUS PAPERS OF THE PICKWICK CLUB, two vols, with colour plates, London, Chapman & Hall, 1910; REYNOLDS (F), THE PICKWICK PAPERS BY CHARLES DICKENS; pictorial red cloth, The Westminster Press; BETJEMAN (J), SUMMONED BY BELLS, John Murray, 1940; TOLKIEN (J.R.R), THE HOBBIT OR THERE AND BACK AGAIN, fourth edition, 1978, with a reprint issue of the Lord of the Rings series and a copy a Tolkien bestiary
I know the 4th impression also from 1974, can be in a box or slipcase. Perhaps both impressions of that year were equally spread between both types?
Yeah, seems possible. So much for my 4th imp. '1st thus in box'! Of course, can't really make a proper judgment from just one example.
I'd say it is more likely that there was just the odd third impression kicking around at GA&U and perhaps one or two ended up being boxed during the 4th run. Hard to see any plausible reason there would have been equal boxed/slipcased runs of the third, given we know they transitioned on the 4th.
Of course, we also don't know if this copy is in its original box.