The photo below shows
location, size, and part number
of posi-drive screws holding the outer gearbox cover of a 1969 Triumph T120R.
Note that posi-drive screws require posi-drive screwdrivers. Use of a phillips screwdriver on a posi-drive screw head damages the head's extra points. A posi-drive screwdriver gives a superior tightening grip without damaging the screw head. A posi-drive screwdriver for the screws on your Triumph's gearbox, primary chaincase, and timing covers is a modest and excellent investment.