All of the 50s lightweights I have have hand painted striping. This would include fork, guard and frame striping. As you move into the 60s it looks like previously hand painted striping like fork darts moved to decals as they scaled up production. They also seem to do away with any hand painted striping on guards and frames (I am speaking specifically to lightweights like travelers, racers, etc). The high end bikes like paramounts still had hand striping.
How they did it? Assembly line. There were stations were workers only did striping. If I remember correctly I saw a video showing someone back in the day striping very quickly as parts moved a long. Correct me if I am wrong.