If your end of allowed stay is before or on 30 September, you can ask for an extension.
If your end of allowed stay is after 30 September (that will obviously be the case of people entering the kingdom from 3 July onward) no extension request accepted. Period.
Every STV holder who is permitted to stay in Thailand any date up to 30 September can request an extension by going to the immigration office before or on 30 September.
People allowed to stay from 1st October onward, will not be eligible to request such an extension.
Therefore, every STV holder who enter Thailand from 3 July onward, will not be eligible for an extension of stay, because they will be allowed to stay at least up to 1 October (90 days) which is after 30 September.
I know perfectly that when you request for an extension you give them all the detail about your address, they put the data in their computer and the system assume you reported yourself and reset the 90 days to day 1. So please people, stop saying that immigration rules are nonsense when you are those stubborn who doesn't want to understand.
I posted links to help people searching what they want to know. And after living more than 35 years in the realm, I MAY know better, especially when I had in several jobs dealt with the Thai immigration.
A 60 days extension type doesn't require to report in 90 days. As you said, the point is immigration give the slip "routinely" putting the farang mind in the blur of what they should do. A proof the officer was wrong is that the guy didn't get a slip when he got his second extension, because he met a smarter officer.