As I see it:
Owner #1: "My airplane needs a new cylinder, I have the parts here."
A&P: " Don't touch anything, I'll be there on Monday and fix it"
Owner #2: "My airplane needs a new cylinder, I have the parts here."
A&P: " well go ahead and pull the cowling, and have the parts ready when I get
there on Mon."
Owner#3: "My airplane needs a new cylinder, I have the parts here. re-man from
Gibsons."
A&P: "Well Fix it then, and don't forget to check the valve to guide clearance.
I'll be there On Monday to sign th' logs. If you run into anything call me"
3 different owners, same problem, same A&P giving "in person supervision necessary...."
I think it's all up to the A&P and his/her assesment of the owners abilities.