In the top center of my 74 Fiat Spider dash is a black, rolling knob, missing it's bezel. Not in my owner's manual. What is it? How to order a replacement? First time in forum, not sure if photo attached

It might be a dimmer for the panel lamps (night-time illumination of gauges)
also sometimes called a panel light rheostat (rheostat =variable resistor).

I couldn't find this item on the usual websites in the U.S. that supply parts for the 124 Spider (e.g. midwestbayless, vickauto etc.) so for a replacement bezel it might be a case of s/hand or modify something else to fit, maybe a window or blower fan switch bezel?

Al.

Thanks, I'm thinking photos I've seen with those are of later model spiders. The right knob in that panel is a dash light dimmer???

As far as I know, yes.