Patented in May 1872 by Orril R. Chaplin, this simple but precise depth adjusting mechanism  uses but a single moving threaded spindle cast with a rather large thumb plate.  The cutter is held to the frog plate by friction when the clamping cap is tightened securely.  Pushing the thumb plate to the right advances the cutter allowing for minute adjustments of  depth of cut.

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The plane was produced in standard bench plane sizes but also included a series of model makers size 0, 1/2, and 1 and were advertised as "Block "planes.  The line was manufactured under the  Tower and Lyon company name by famed gun maker Iver Johnson, who subsequently made a number of later patented modifications to the design including the more common lever cap tightening system, an adjustable throat and an India rubber tote patented in 1888, 1899 and 1902  by Johnson and Reinhard Torkelson. 

 

This exceedingly rare No 2 example is an early type produced c1876, measures 7 inches in length, 1 7/8 inches wide with a 1 1/2 inch cutter.  The plated cast iron  tote is dovetailed into the bottom.