The JZ was designed with the Italian market in mind and at that time all Italian rear plates were rectangular...
The cars originally come with a black plastic part that fills the recessed area to a flat, vertical, plane on which the plate was mounted and finished off with stainless steel trim, like in the pictures of 40-16-ZS and 53-39-UR and is probably the same part that Mike is talking about.
This is not uncommon as most Alfa's with a recessed rear side had such a plastic part (i.e. the Berlina and probably more).
It is shown in table 118 of the parts list:

Partnumber: 105.93.59.038.00/01 indicating that it is a JZ specific part (hence difficult to find). Proper Italian name: Supporto cornice targa.
BTW, there is a third variety of Dutch plates which was rather short lived and only valid for cars which got a Dutch license between 1976 and 1978... it is called light blue and is actually a couple of shades lighter than "old blue". It can still be ordered as such from specialist licenseplate manufacturers in The Netherlands.