Hi, The piece you have purchased is the panel that goes between the outside of the spare wheel well and the inside of the rear fender under the trunk floor and spot welds to the underside of the trunk floor at the top and spot welds to the inner lip of the fender at the bottom and to the fender well seam at its front. It creates a cavity that is very good for collecting dirt and moisture and growing rust.
It is also the only type I have seen and would provide more strength than what you show if in fact what you show is located in the same place as I described above. Other than to collect dirt that plate is there to transfer trunk load to the lower chassis and interestingly Alfa did not do something similar on the other side.
It could be that early GT chassis had your version and this stronger, longer version was an update. The 1967 GT, GTC, GTA parts book is silent on that part as far as I can see, so some one with such an early GT chassis as yours may chime in.
As to whether a concourse judge would catch such change, I would doubt it, Most will not know the cars that well, nor will they want to get grass stains on their white pants crawling to look, IMHO.
Ken Geiger, Toronto
1965 GTA, RHD, Stradale
ex- 1965 GTA, RHD, Corsa, Trans/Am 66-72
1964 Sprint GT, B-Sedan in 70's
1970 Giulia Super (CDN Spec)
1976 1600 GTj (Italian Spec)
1966 Harley Sprint (Aermacchi) 250
2000 Catalina 320, Hull #765 Northern Dream
2001 BMW Z3 3.0i