diverter stem

I have a diverter that is leaking when its in the "shower" position.

I took the diverter apart and I got to the stem, however, the stem is nothing I have seen before, I don't see any hex to get the stem wrench on.
There is two flat surfaces on the brass, i tried to take pliers to it, but it didn't budge.
There is a nut on there as well, I thought maybe if that comes off, which it did, I could slide the guts of the stem out and replace the gaskets.

any ideas guys?

Thanks
The house is 50-60 years old, if that helps

I took the "just force it" approach. I used a bigger wrench on the brass part with the flat sides and got it to budge, and then after a full turn it started to move freely using just my fingers.

Just as an FYI for anyone with the same issue.