The frame which supports the seat frame had a triangular sheet of armour plate at the top, onto which the head rest was attached. A slot below the headrest allowed the straps of the seat harness to pass through, where they were attached to twin Bowden cables, which ran back to the anchor/pulley in the rear fuselage.
A rectangular sheet of armour plate was attached, behind the seat, to the seat frame and fuselage frame, the top of which sat just below the lower edge of the head armour. These are clearly visible in all wartime pics of all Marks of Spitfire, although there are minor differences in the head armour, Mark to Mark.
I haven't looked at the Tamiya kits in my stash for some time, but, from memory, these parts should be included - certainly the head armour at least.
Many kits are modelled from preserved, often airworthy examples, and this can lead to errors, where details are missed, or sometimes added (!), to duplicate the original subject. The new Revell 1/32nd scale Spitfire MkII is a good example, being modelled from an example at Duxford, and lacks the back armour, just like the full-sized example used as a pattern, which lacks the armour to save weight, and allow easier acces behind the seat. This kit also duplicates other features of the 'donor', and is more like a MkVa than a MkIIa !!
If only the researchers would ask a few questions when measuring the original, it would save a lot of hassle. The canopy on the Bf109 E4 is another prime example where errors very often occur, most being modelled on the RAF Museum example, which has a replacement canopy from a Buchon !!