Found this bottle. Can someone tell me what it does
 

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Found this bottle in the back of my 86 fc. Don't know what it does was empty when we got it. Car was sitting for a while. I can't find anything online about it, pleas help a brother

It was the reservoir for a cold start system that Mazda realized did not work and recommended owners remove.
It was supposed to hold antifreeze that was injected into the intake because...reasons.
It was a silly idea because...reasons.

Sub-Zero Start Assist Reservoir. Intended purpose was to put coolant in the combustion chambers when it got really cold out (0 degrees I believe) to help start the engine. In simple terms, it didn't work, so Mazda issued a TSB to remove them. I still have mine in a box somewhere. If the material it's made of can reliably handle oil long-term, it'd be an awesome 2-stroke reservoir for anyone that goes with a RotaryAviation OMP adapter. Future mods... :D

Thank you everyone for helping me. I really appreciate it.

I've always just left mine in place. The Sub Zero Starting Assist was a package that was installed on all RX-7's with the Federal emissions package. The California emissions cars did not get the sub zero starting assist. It uses an oil temp sender that is threaded into the front of the oil pan on the driver's side. CA emission cars did not have this temp sender of course, let alone the rest of the system. Once the oil fell to a certain temperature, the system was supposed to inject a 90/10 mix of anitfreeze and water into the intake to deice the intake on start up. Mazda has been doing this since the RX-7 was introduced in 1978 as a 79 model.