I can’t see anything refering to a thread on that section of drawing.
It’s all relating to a gear, 50 teeth @ 16 diametral pitch with 5/16″ face width (effective width of the gear).
Found a calculator here https://evolventdesign.com/pages/blank-calc?srsltid=AfmBOoobxsgp7bOTdcdnGaOVADYR222NDQDWhikOLjACyVzdLvpKHGvL
Number of Teeth: 50
Diametral Pitch: 16
Pressure Angle: 14.5 degrees
Gear Type: External
Outputs
OD Reference: 82.5500 mm 3.2500 in
Pitch Diameter: 79.3750 mm 3.1250 in
Root Diameter: 75.4063 mm 2.9688 in
Addendum: 1.5875 mm 0.0625 in
Dedendum: 1.9844 mm 0.0781 in
Working Depth: 3.1750 mm 0.1250 in
Whole Depth: 3.5719 mm 0.1406 in
Circular Pitch: 4.9873 mm 0.1963 in
Tooth Thickness: 2.4936 mm 0.0982 in
Base Circle: 76.8467 mm 3.0255 in