Not being a "poor boy" expert I can say I'm not surprised at your findings.
Poor Boys are not noted for their excellent workmanship and fancy features. They are a common, everyday, working rifle who's reason for existence is totally functional.
That said, there are gunsmiths and there are gunsmiths.
Some just do a job and some have pride and want to do their best even though the final product will be plain.
I suspect this explains the various methods of installing a side plate.
One gunsmith will just say, "Hell, I was thinking of leaving it off altogether but I thought it might make the gun last a little longer without the screw eating into the wood there."
Another gunsmith might say, "A good gun will have the sideplate inlet into the wood so it strengthens the stock in its weak area so I always inlet mine."