Actually, Dick, this is an interesting question.
The 2004 GCR/ITCS noted on the ITA and ITS Miatas that "Detachable hardtop shall be installed. Latches shall be replaced..." (my emphasis). However, the 2005 GCR/ITCS states, "Detachable hardtop may be installed. Latches shall be replaced..." (again, my emphasis).
This is a significant change, without any support via Fastrack or CRB notes. Further, the change in the 2005 GCR is not noted with any change bars (of course, now that I look it appears none were used in the 2005 GCR anyway.) I've asked a couple of folks about this, specificaly if it was an intentional change or a typographical error, but as of right now, as per the 2005 GCR, hardtops are optional in Improved Touring... - GA