In the UK and english speaking world generally, a Stall Turn is a vertical climb continued until flying speed no longer can be maintained, full rudder is applied to swing the aircraft about its "Z" axis to then face vertically downwards. A Hammerhead involves going backwards, erect or inverted (positive or negative).
The hammerhead does not involve a stall, and does not involve a tailslide.
Wilipedia:
1/4 loop (pull or push) to vertical, as momentum/airspeed decreases, rudder is applied and the aircraft rotates around its yaw axis, the nose falls through the horizon and points towards the ground, a momentary pause is made to draw the vertical down line, and 1/4 loop to level flight. This figure is sometimes called a stall turn which is a misnomer because the aircraft never actually stalls. The manoeuvre is performed when the aeroplane decelerates through 20 - 30kts (more or less, depending on the aeroplane flown) of airspeed. The cartwheel portion of the hammerhead is performed with full rudder and full opposite aileron. Gyroscopic forces from the propeller during the rapid rate of yaw will produce a pitching and rolling moment and a degree of forward stick will be required to keep the aeroplane from coming off-line over the top. The yaw is stopped with opposite rudder while the ailerons and elevator remain in position, then once the yaw is stopped and the aeroplane is pointed down vertically, all controls are returned to neutral together. Although they can be flown left or right in any aeroplane with the proper technique, a hammerhead is best flown to the left with a clockwise rotating prop, and to the right with an anticlockwise rotating prop (as in a Yakovlev type), due to propeller torque/gyroscopic effects.
British Aerobatic Association on stall-turns:
http://www.aerobatics.org.uk/judging...tallturns1.htm
http://www.aerobatics.org.uk/judging...tallturns2.htm
Incidentally, so far as origins of aerobatics, the first aerobatic display was at Rheims, France in 1909. Not until 1913 did Adolphe Pegoud begin displaying and performing aerobatics on behalf of Louis Bleriot. Lincoln Beachey began flying aerobatics as an exhibition pilot in the United States in 1911.