Taking a quick glance through Derek James' Westland Aircraft since 1915 (Putnam), it states that the early one piece canopies collapsed under loading and were strengthened, although I suspect this entailed thickening the perspex rather than the metal rear section - read on.
Further along under the heading of Wyvern S.Mk.4 it states that "As a result of squadron experience a number of modifications were introduced in the Wyvern after it had entered service. These included a change to a Martin-Baker Mk.2B ejector seat, the use of a composite canopy with a metal rear end to replace the stiffened one-piece type..." So, it looks like it was a mod introduced post service entry in S.4s. There are pics in the book that might help, S.4 VZ752 of 813 Sqn and four of its stable mates are shown without metal rear canopies, but the other serials are obscure. VZ748 the third production S.4 is shown with the metal rear canopy. The thing that is needed is to establish when exactly this mod was introduced and to what aircraft within what time frame the mods were done.
The S.2s converted to S.4s are recorded as VW870, VW873, VW880 through VW886, so nine of the 20 pre-production Mk.2s. There's a picture of VW880 in the book, but it has the clear rear canopy.