Apologies, you’re referring to the second picture. I think that looks far too small for a soil pipe based on the image. However as you postulated previously, the property was (possibly), highly deficient in build quality so it may be wise to take precautions. What is the width of the pipe imaged in the second picture? Is it possible to refer to US building regulations for the necessary width of soil pipes for that era, in order to confirm accurately so you don’t cover your face in faeces?
In my humble opinion, I think the second image relates to the water feed pipe supplying the property. Obviously I’m English and have every clue about British regulations but none about American. So if I’m mistaken I’m sorry. Cheeeers
I learned from my next door neighbor that his Main Drain Cleanout is in the back corner of his home, ours being built at the same time and likely by the same developer. Going to do some 'water witching' come next day it's not a deluge.