Looking at Paul Bartlett's site, wagon pages, I think 278018 is likely to be a Diag 187 van, others of this batch being numbered 277676, 277977, 285119 etc. These were plywood sided fruit vans with diagonal strapping either side of the doors. The ends were corrugated for the lower half only, the upper half being (I presume) plywood as well.
To muddy the waters so to speak, there appears to have been a few Diag 195 vans produced within the same number range, ie; 283710, 283799, albeit these seem to have been almost identical with the exception that the ends were completely plywood.