[Mr70]

If you're talking about the single one piece unit,that was New for 1970,that combined the in dash AM/FM 8TK unit as a Stereo radio,not a Mono.
It needed 4 speakers to reflect this.
I didn't think the Nova dashpads had 2 corner speakers to take advantage of this,so it sounds like they installed kickpanel speakers,along with installing two rears in the package tray provisionals,to let you experience the full 4 channel effect.
Both the radio station and 8Tk tape mode would be heard through all 4 speakers.
The center speaker was for Mono radios only,so this stereo unit would have ommited using that 5th center speaker originally.
Both front & rear were 10 Ohm speakers.Fronts being 4X6,and rears 6X9.
As stated,RPO NA-9 aka EEC,(Evaporative Emission Control- aka Charcoal Cannister),equipped gas tanks were for vehicles that were sold BRAND new in the states of Calif.and certain areas of Nevada.
Not at all related to TCS,(Transmission Controlled Spark),or SMOG aka A.I.R.(Air Injection Reactor).