My parts book only gives Fleetline serial numbers for the Carter UT carb. It does show the Marvel Schebler for the Super 66, and doesn't give a serial number range for the Zenith. Looks like it could be either, but the MS model is TSX 603 and the Zenith is 12427. They went to the MS TSX 775 on the Oliver 660, and the Zenith might have been used then, the book isn't clearest on that.