Tach Reading Faster Than Tach/dwell Meter?!

Okay, so the adjuster on the 280 tach is the aluminium thing with the slot that has 30K stamped on it, and the 240 is the brass nut.

Yes, they are both trimmer pots.  The 280z is the conventional circuit board mount but the 240z looks like a standard potentiometer on the bottom (black) that is mounted to the board by the shaft (brass nut).  These appear to be turned from behind through a hole in the case.