I'll Lock Up
It's actually such an easy fix that it can easily be done at home. All you need is a needle and thread. Here's the 'how to' :If you are buying vintage you very well may get a jacket where the elastic on the action back has worn out. i had this problem on a G-1 I purchased and it was an easy repair for my tailor, as @Aloysius said.
Having never used a sewing machine, I would imagine they you pull the leather taught along with a certain amount of stretch to the elastic, and sew. How did they make them in the first place, I wouldn’t know of course. Now I'm curious too. I often just like to imagine how I'd go about fixing...