About Audio-Technica headshell with fixed screw holes:
Yes there are no slots to move cartridge in headshell but on the backside of the AT Technicard or Magnesium headshell you will find at least two sets of mounting holes. The most important thing about this type of AT shells is FULLY ADJUSTABLE OVERHANG AND AZYMUTH and with thus option you don't need any slots, you move the whole shell to adjust cartridge with your protractor!
Just a note: not all AT magnesium headshells are adjustable. The mg 10 is not. I learned this the hard way.