The SHX marking is a single persons mark, initials, or ? We have also seen him/her mark in other areas. I have photos of the SHX behind the side scoops on a car. Pretty sure some where else. With that said, I am confident the SHX is a rogue employee with a grease stick. I am confident it has nothing to do do with the topic at hand with regard to tires.

I am assembling a post on the 70X marking we see so much of to answer your other questions but it is past my bed time so stay tuned tomorrow....

D

EDIT (5/7/22): Not a rogue employee for the "SHY" marking. The term "SHY" shows up on the final line inspection document. Meaning unknown at this point.