I believe I recognize that problem. My throw-out bearing did the same thing EXCEPT the piece that broke off the collar went into the clutch assembly taking out one of the hub springs which then took out my pressure plate. Had'nt put much mileage on the car and $400 went right out the window.
This is what I found. I purchased a replacement aftermarket
(not original)GM clutch fork that was supposed to work. Nice heavy duty piece. Fit the throw-out bearing well & nice and tight. Snapped right onto the clutch ball in the bellhousing. Clutch linkage all lined up and fit as factory. Yep, thought I had something that was going to be around for a good 200,000 miles.
Here's the problem. The area on the clutch fork where it slides over the throw-out bearing appears to be generous enough, but once you clip it to the clutch ball, it positioned the inside edge of the clutch fork too deep onto the collar of the clutch bearing. The linkage pushes the clutch fork, which pivots on the clutch ball, acting like a fulcrum/lever and......... the inside edge of the clutch fork does not have enough clearance to pass by the collar and hits that collar with all that leverage and breaks it off. It is hardened, so it does not bend.......it snaps off. From your photo, looks like you nipped yours several times. Mine just took off one big piece, like about 1/3 of the collar and did all my damage.
The replacement fork is generic. DO NOT USE IT!. I found a junk yard out west that actually had a '68 Lemans 4Dr with a factory 4-speed parts car. He still had the clutch fork, so I bought it very reasonably -because at the time all you could get new was the aftermarket "supposed to work" GM style clutch fork. Problem solved with the used '68 fork and setting them side by side you could see the difference in the pocket that held the throw-out bearing.
Now available is a factory correct aftermarket replacement clutch fork. I purchased one in hopes it might be as factory as my original '68 clutch fork. Sure enough, a perfect match in all dimension.
I can't seem to find my receipt for the clutch fork, but I believe I got it from the Original Parts Group catalog. It sells for $79.99. I am sure other Pontiac parts specialist have the same part and perhaps cheaper. You want the one that is factory correct.......and nothing less.