What is the part called that chainsaw bar flange nuts screw into?
They are generally called "bar bolts". These bolts are often pressed into the chain saw housing from the back side, it can be a complicated repair requiring significant disassembly of the saw.

Also, I do not see them on the parts diagram. This doesn't mean they are not available, but to find out you would have to call Ryobi customer service and explain what you need.

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