I know this is a long shot but I thought I’d try. I’m looking for the brake fluid diaphragm that would fit a GRX810 shifter. It looks like Shimano uses the same part for recent mechanical GRX, Ultegra and 105 shifters.
See link for an idea of the part. Y0C578000 The shimano part number appears to be Shimano ST-R9120 Lever Oil Diaphragm, Fits R8020/R7020/RX810/RX600/RX400
Ideally this would be from a broken shifter in someone’s part bin, but I’d be happy to buy a left GRX810 shifter if needed. I can order one however I actually need to get this before next Wednesday as my son will be at a training camp
Bike shops can order, but the lead time is a bit of a problem.