That Wikipedia page goes into detail on why jet injectors are no longer used, but there is no mention of the scarring. It used to be common to see people with a circular scar on their upper arm where they got the injector shot as a child.
https://www.verywellhealth.com/smallpox-vaccine-scar-7096798
Edit: I asked around and nobody seems to know what I'm talking about. Seems either it was a local peculiarity or a misremembered childhood.