That's part of the throw out bearing assembly. If you buy the newer-style throw out bearing assembly, this part gets replaced, as you'll see below.

In the transmission bell housing is the throw out bearing actuator arm. At the inner end of the arm is a round clamp that holds the carbon disc (the throw out bearing itself). Here's a new carbon disc (from the Filling Station's web catalog): https://store.fillingstation.com/detail/AF-91/Chevrolet_19271928_CLUTCH_THRUST_CARBON_DISC.html

Here's the newer throw out bearing assembly, where you can see that the carbon disk has been replaced with a ball bearing: https://store.fillingstation.com/detail/CV-102/Chevrolet_192531_THROW_OUT_BEARING_CONVERSION.html

No machining is required to use the new bearing assembly. ;-)

Cheers, Dean

Dean 'Rustoholic' Meltz
Lurch: 1927 1-Ton Chevy Cattle Truck
Old and ugly is beautiful! -- The Saga
Lurch's Gallery
Justin: 1928 Chevrolet AB Canopy Express
In the Stovebolt Gallery