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Anyone know where I can get a replacement Yale type P119 sash bolt key
like the one in the photo below?

I could go and buy another packet of six sash bolts that are supplied
with a single key, but the key is made of soft metal and breaks
easily[0], and I still have six spare sash bolt sets from the last time
I did that.

http://i65.photobucket.com/albums/h222/bofphoto/yalekey000288.jpg

Or even better something like a socket, made of decent steel, I can put
on the end of a driver handle? The square head on the pin is approx.
3.5mm across the flats and the outer dia. of the key is about 6.5mm.

That's about the size of a No. 5 clock key.

Clock keys are readily obtainable (e.g. on ebay) in spider sets --
usually one set covers the odd numbers and another the even numbers.
Once you establish the size that fits, buy a single key of that size.