Benjamin Grieshaber initially patented a hump filling pen in 1910. According to that patent, the lock for the hump functioned by rotating the section of the pen. This pen does not seem to have been produced. The pen known as the Grieshaber hump filling pen is a more elegant design, citing the Watts patent 1020327 from 1912. There is no outwardly visible lock for the hump. The hump is notched at one end inside the barrel, and a locking device is rotated internally by means of an end knob. A tube, cut on an angle, is attached to the knob. It rotates so that the longer end of the tube engages the internal notch thereby preventing depression of the hump. As little as a quarter rotation moves the longest part of the lock out of the notch, freeing the hump to be pressed and compress the sac.