So, the way it works is the following: on that place on the back of the shaft there should be a thumb knob which screws onto a screw stud which in term tightens a clamp around the shaft that locks its movement. There are no threads on the walls of the hole and in my case the crew was ripped for over tightening (I guess). I learned all of this by going to a Mariner distributor and talking to the parts guy.
So, I'll have to order a few parts (clamp with the screw, spring, washer and nut) and then open the shaft to install.
For now I'll tie with some rope to keep it somewhat near midships.
I had to go look, Sorry No Mercury lable.
4.5 & 15 johnson is on the port side hex head slotted screw.
5 Nessan is on the back plastic wing bolt, with a spring.
I think you had 4 pictures with a extrusion on the rear I don't remember seeing a hole.
if you run a seach for your engine there is a company that has the catalogs for both my Johnsons, covering every part for free. You have to have a part number to buy the part.
Don't forget fasteners need to be lubed, & don't mix lithium & sodium grease. (The chimical formula for mud.)
PS I would like to thank you for the question, When I looked I realized my son had been wrong, about the adjuster on my 15 being broken.