Sprint part no. is 110621 pretty sure it is 1/4". Doing a bit of digging seems to be equivalent to 134591 as used on TR2/3/4/4A , but please double check before parting with any hard earned! This available from some brothers in Lincolnshire:
https://rimmerbros.com/Item--i-134591
Can you measure the width of the slot and the depth and also the depth in the hub to give you the size? You could then speak nicely to the Lincolnshire people and get the size checked before ordering.
As far as I know keys are not hardened, they seem to deform quite easily when trying to get them out of shafts when they are rusted in!
On a pedantic note, pretty sure that technically Woodruff Keys are the semi circular ones, and this is just a straight key, though most people do refer to them as Woodruff Keys. Only mention it as I do have a selection box of "proper" Woodruff keys and was about to go measuring up when I looked into it more closely
Hope that helps
Roger