Sorry Andy, I didn't word that very well. Yes, the reinforcing strips were field mods, sometimes done at squadron level, more often at an MU or during a deep service programme. Production mods were, if I remember correctly, the skin thickness and,I think, internal stiffening across the wheel well itself. I could get the full details I think, but my wrists and fingers can't tackle the weight of the Shacklady/Morgan book at the moment!
The way I look at it, as the field mod doesn't seem to have been carried out 'across the board', then with mid to late production serial blocks, unless there's evidence to show the stiffeners on a particular aircraft at the time being modelled, I'd omit them. These were sometimes added late in the airframe's life, for example after general squadron use, if the aircraft had been passed on to an OCU or other training outfit. A 'real life' example of this is the MkV AB910, now with the BBMF; this aircraft has the strips, and around 1982, when I was doing the AV production at the 'Flight, I was curious about these, as I hadn't really seen them before, or at least they hadn't registered fully if visible on period photos of MkVs. When I enquired about the strips, from what I remember, I was told they were a later addition, I think as late as 1943, when the aircraft had by then seen some service.
As EN921 was built in spring '42, it makes we wonder about the photos you posted, as one shows the earlier, MkI/MkII type of windscreen, with external armour, which had been replaced by then, whilst the other has the internal armoured screen. I wonder if these are two different aircraft with similar nose art?