My response is not simple because how AC Cobras (leaf spring chassis) were outfitted was not simple. (If you expand the discussion to 427 Cobras another variant would be included.)
Part 1: There was not a single mirror design and or variant used on cars sent to America but several. Just examining pictures published in new car road tests and by Shelby American’s in house photographer of the period you will find at least three easy to spot different assemblies. You will notice the type of mirror used on chassis seemed to be in batches with very early cars using one style, middle and late cars using one of at least two variants. On later cars one variation in mirror assemblies was just painted versus plated glass clamps. As far as I can tell all O.E. AC Cobra assemblies included the “LUCAS No. 104” stem.
Part 2: I personally am not aware of an exact recreation of any of these O.E. mirror assembly designs.
Part 3: New old stock mirrors of all varieties show up for sale occasionally. Just to add more confusion, there were curved glass mirror assemblies and short mounting stem versions used in some other applications that people buy mistakenly thinking they are correct for a Cobra.
Dan
