The 73 z1 fuel valve uses a rubber sleeve between the fuel valve body and the fuel valve lever, the design changed in 74.
I bought some in 2003 and possibly later, now everyone's memory has failed.
Who made these in 2003 ?
I know what you're looking for and it's the one part of the petcock that I have never seen for sale. You can replace it with one from a repro petcock but it will probably leak. This is the reason many Z1 owners have gone to non stock petcocks.
I ride Z1's. I have a couple of '75's, plus an '80 KZ1000LTD. My lovely Sweetums rides a 1981 KZ550.