As a "factory standard" Porsche installed the speedo on the left, and then later moved it to the right in mid 1957 along with the T2 body changes, it remained on the right through the end of the T6 356's. So to be "factory correct" if you have a beehive car it should be on the left and if you have a teardrop car it should be on the right... I don't know where it was installed on the "changeover" cars that were a hodgepodge of parts...
As for the replicas, yes, we chose the left initially because it was the best route for a stock VW speedo cable, and often times they stay there out of habit. With our integration of the GPS speedo, we moved them to the right to be more "authentic" but people like to spec every detail and Ive had more than one person ask for it on the left in a teardrop car.