The early 850 Spider had an optional 3-point (fixed, non-inertia reel type) seatbelt, that's why there's always been a threaded upper mount for it (next to the convertible top pivot bolts). 2-point lap belts were standard issue.
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As for the side mirrors, early Spiders had a mirror fitted to the top of the driver-side front fender (just forward of the A-post), later cars had a different type of mirror fitted to the driver-side door, instead. These were standard factory parts. A passenger-side mirror was dealer-optional on the later cars (same mirror as the driver-side one).