Went shopping and found Cub Cadet (CC) has changed the part number and drawing to show the pulley and a key, and a note saying to use it "with" another part number which is the key. So it looks like BigFish nailed it as being no good and to replace it, as CC changed it too.
Oddly for once the CC site has it priced in line with aftermarket.
At checkout it had Kohler in the invoice short description. Mine is a B&S so I had to reverse and confirm to find I picked the right part, they must be similar and CC migrated it to the B&S drawing. We will see what comes in the box.
I'll post back
Could not get a better key pic. It looks to be brazed and has a copper color, maybe a silicon bronze filler rod was used.
The Mower, a CC 44" Cub Cadet Z-Force 18HP B&S being replaced by a 23 HP B&S soon. I added the duplicate fuel tank on the right.