Didn’t Westland develop the low back Spit?

Mk21 took longer to develop because of the new wing and had handling problems to sort out. Also, I suspect it was a case of various different marks being in production at the same time (IX, XVI, XIV) and the low back being introduced during production. Other marks like PR XIX not strictly ‘later’ because they were developed from earlier versions. There was not one single, straight line of evolution in the Spitfire family but numerous ‘branches’ and I suspect this as much as anything is why some variants seemed to be ‘throwbacks’.