Throat adjustment mechanisms consist of a “shoe” (or slide, or mouthpiece) which normally sits in a cavity under the plane toe, a finger knob, and some integral mechanism to facilitate adjustments to the width of the throat opening.
Adjustment mechanisms for regulating the width of the throat opening in planes had been around since approximately the 1860s, albeit in the form of intricate systems for wooden planes. Consider this patent from 1870, showing a throat adjustment mechanism. Here the metallic shoe is held in place by a vertical screw, and adjusted through the toe of the plane by means of a horizontal adjustment screw. This would likely have worked quite effectively.
In 1888 a patent was granted (No. 381,141) for a plane throat adjustment mechanism based on a sliding “T” shaped piece to adjust the throat of a corrugated sole bench plane. The earliest throat adjustment mechanisms for block planes consisted simply of a knob and throat adjustment shoe. The throat would be adjusted by loosening the knob and moving the shoe along a slotted screw hole.
In 1894, Traut and Borner received a patent (No. 515,063) for what was to become Stanley’s core mechanism for adjusting the throats of block planes – the eccentric lever. This patent would morph into the actual mechanism found on planes. The earliest eccentric lever had two distinct features: a lever tab for the user to move the mouthpiece, and a pivoting post attached to the plane toe, behind the finger knob.
In a Stanley catalog from 1898, both these features had changed: The lever tab had been turned down (except in planes where there was no room for this), and the pivot pin had been attached to the eccentric lever, such that the pin fits in a hole in the plane body. In many Stanley block plane patents after 1900, the lever tab shows in the turn-down position. In Catalogue No.14, all four of the “Bailey” adjustable block planes have the lever tab turned down, similarly the No. 65 and 65½. The No.60 and 60½ however, have the lever tab turned upwards.
Two examples of planes with the pivoting post integral to the toe of the plane body are shown in Fig.1. The Siegley “No.18” has a unique integral post construction, more similar to the original Stanley patent. The No.S18 also has an integral pivoting post due to the thin nature of the material on the planes toe.
The next two photographs illustrate the pivoting post integral to the eccentric lever. Also shown is the eccentric lever tab shown turned downwards vs. upwards.