That is woodruff key stock. I am not sure it goes anywhere in the front hub assembly. I had one apart down to the spindle (didn't need to take it off for what I was doing) and there are no keys in it at all. The groove you have it laying in is for the tab on the washers to slide into and hold the washers in place, it really serves no other purpose that I know of. I have not had the spindle off but I do not believe it has any place for a key either.
I just double check a pictoral break down and it shows no woodruff key in the spindle area either. Where did you get the info that you needed the key stock? There is one in the rear, I wonder if someone was thinking rear when you said drums?
Careful your character's showing
CSP is correct,
The 'Slot' in the spindle is for the keys on the washers so they don't spin when you tighten up the nuts.
Without the keys in the washers, the washers can spin, causing you to over tighten the nuts and more importantly,
Allow the turning bearings to LOOSEN the nuts during operation.
Your spindle is shot, let me know if you need one, have a 2 or three laying around here and you can have them.
Just PM me an address and I'll get them out to you.