I bought a PX-10 new, in France, in 1970 and it came with a 100 mm stem - which seemed to be about the most common size on the French "Racing" bikes that I recall. The stems on bikes with less Race aspirations, the more common recreational rides, had slightly shorter stems.
I think these mid-length stems (90-100 mm) were all just left on their original bikes... which are now in landfills or thrift stores.
Photo below shows the typical recommended set-up for saddle and stem in the 1960s-70s - which also explains the lack of long seatposts.
...Notice the bike in the photo appears to be a Peugot UO-8 
