yup, the second one is a back flow. Notice the angled access. Also note that it is all plastic, not cast. Need I say more, no, I don't think so. Once again, proving your ignorance.
Das, he's an idiot, he just keeps proving it over and over again. Yes, the pic you posted is a clean out. It is the conventional. Most cleanouts are at the bottom of the stack to give easy access to the main. Not all houses have a back flow. Newer houses, yes. Older houses, no. We install back flows all the time on older houses. It is NOW a code requirement. It was not in the past. The angled port on the top of the back flow is a clean out. It is required so that you can auger the main without having to try and auger through the back flow and possibly get your auger stuck. Houses that do not have back flows, and I will bet the house that dumbdumb lived in didn't have a backflow will have clean outs(note the plural) in different places possibly. Including, but not limited to, the bottom of the stack and in the main, possibly buried in the floor. These clean outs can be a straight T but should be a Y angled towards the downstream side.