Dario,
Are you sure there were Speedster GT's with factory installed mirrors? I'm not saying you couldn't order them from the factory but I always read and heard they were dealer installed. Do you know of any Kardex that lists the mirrors? From 1959, according to Dr. Brett Johnson, some 356's, but only those delivered in Europe, had factory installed mirrors but I have never seen evidence that it also happened on the GT Speedsters. On all the delivery pictures I've seen, the Speedsters GT's are without mirrors. How else could you explain the difference in mirrors (GT, Spyder, Aero, Ponto Stabil, Talbot) even when only looking at the last 32 GT Speedster made in 1959? When you look at this page home.tiscali.be/carrera-speedster/59.htm , you'll see a picture from 1959 of Speedster GT #84934 which the original owner sent to me. It doesn't have a mirror. Many Speedster GT owners are restoring these cars to "factory condition". As a result, they don't have a mirror. See #84927, #84933 and #84954 on that same page as examples.
btw I hope you sell lots of those GT mirrors because they just look great even on non-GT Speedsters.
Johan
Porsche 356 Carrera Speedster Inventory
home.tiscali.be/carrera-speedster