Originally, all Glocks were made in Austria. There was no established distribution channel established as Glock was entirely new to the firearms business so early guns were all built and assembled in Austria and then imported to wherever they were to be sold. It took almost three years for any real traction to be made in the US, but it soon became clear that this was the ticket to massive expansion. Information is hard to find, but we know that by 1986, the Glock 17 was regularly being imported to the US. Each one had to get import stamp markings until Mr. Glock realized how much money he could save by establishing a beachhead in the states, bringing in the parts and then assembling the finished gun here.
This led to a unique subset of various models that have garnered quite the collector following 35 years later. The earliest imports all have Austrian proof marks on the barrel, slide, and frame and will all command a premium in comparison to the same model without the stamps.
All but the earliest Gen 1s will have the three stamps (the earliest models were built for European so if found in the states, they were imported later on) and the US specific designated models did not show up until 1988 just after the transition to Gen 2. You will find models 17, 17L, or 19 as the only Gen 2s with austrian proof marks. However, the gun-buying panic that took place in 2008 after Barack Obama won his first election resulted in several Gen 3 models being imported that were originally built for other destinations. These Gen 3 Austrian proofed models are known to exist in models 17, 17L, 19, 22, and 23 but there are likely others out there.