John, if I remember correctly the D.31A spar has additional ply webs on the front and rear face of the spar and would be noticeable from the cockpit where the center section is quite visible. Can't remember what the ply thickness is, might be 1/8" or 1/4". The additional webs only go so far out into the wing.
If you're building a Turbulent from Rollason plans the quality of the prints is such that the wing rib dimensions are confusing as they show the revised D.31A rib which won't fit a standard Turbulent spar correctly as I found out, fortunately before I'd invested too much time and money.