Reo P67 squonkers,, just buy 8ml silicone bottles,, the ones i bought with the fill port are fine,, but the fill port is not a sturdy design,,, I had to seal it up with food grade silicone-----but the little lever to open it up is useful in pulling the bottle out.
As for 510 refill squonker bottle,, i have tried a few,, you dont want delrin threads,, you do want a gasket on the end to avoid shorting out your mod. The last one I bought has been perfect so far,,,, please note some have complained the bottle wears out,,, I plan to repair the bottle with my food grade silicone or convert it using any of my other feeder bottles,,, or find a bottle that fits,,, over time.anyway below is a pretty good product
https://www.fasttech.com/products/9664168