that's a sensing bourdon tube that "automatically" operates the valve to the setting of the lever as water temp varies - GM dropped that later because it was a problem spot, and I don't think any of the repros have it, just some NOS - I'd prefer one without, temperature can be controlled well enough just with the lever opening or closing the valve as required - also be wary of NOS, the critical part is a rubber 'washer' that eventually leaks, likely sooner after 30-40 years on the shelf
Bill
