Hard Dog Miata Roll Bar Application Guide and Info
Regarding your style of convertible top:
All 1990 - 1997 Miatas came from the factory with a flexible plastic rear window in the convertible top. If your first generation Miata has a glass rear window, it's either aftermarket or a second generation transplant. The basic 1990 - 2005 Miata chassis didn't change allowing the tops and roll bars to interchange. A few quick measurements are very helpful when trying to determine just which bar is compatible.
Factory NB (second generation 1999 - 2005) Convertible Top Glass Rear Window Dimensions for comparison:
14" Tall in the center
32.25" Wide across the top measured just below the upper corner radius (approx 2.5" down from the top edge)
Be sure to include the complete assembly including all rigid parts around the edge. Not just the veiwing area.
Width measurements across the bottom of the glass are not of concern.