You are assuming to much. Service station owners cared less about the pump, when I owned a shell station in 1970 I could care less about what was on the pump. If Shell wanted something be put on the pump, I didn't care, nor did I care who made the pumps that were on my island (actually I never looked to see who made the pumps, my memory tells me they probably were Wayne pumps).
Normally, selling gas brought the customer into the station, now what should I do to get his car in the bay where I can make the money it took to pay my help, and pay for all the other things that it took to keep the station open, and maybe there might be enough left over that I could take something home. I didn't and when my dept reached $15,000 I told Shell where to stick their station.
There other assumption is that a pump stood out there for many years. Not true, large oil companies who owned stations changed pumps every four or five years. Think about it, the only thing that hung out there was the air meter, they were rarely replaced, that is why it very hard to find one today. OK, back to the pump, they were traded in when the old pumps were replaced. These were rebuilt, repainted, so if a M&S pump branded Sunoco was replaced, eventually sold to a Shell station owner, Shell would be out there the next day with Shell ad glass and a Shell globe for the pumps.
When I was restoring pumps, there was always one that had many coats of paint on it, each one a different oil company and since the oil companies gave away the ad glass and the globes, any given pump could have many different pieces of ad glass or globes on it.
Your are also assuming that M&S only sold their pump to the four companies you mention. Again, they would sell these pumps to anyone. An example: The blend Wayne pumps, many believe these were only sold the Sunoco, they were sold to anyone who wanted one.
I am not typing this to be critical, I have always said "have fun with you pump." Years ago I invented a statement "There are no pump police out there who will tell you your pump is incorrect (many give DB credit for this this statement, but I was the one who first used it). Again, have fun with your pump.