nber category 6 Others subcat 67 Pipes & Joints

uspto classes
138 Pipes and Tubular Conduits
277 Seal for a Joint or Juncture
285 Pipe Joints or Couplings
403 Joints and Connections

Historic patent counts for subcat 67

uspto search in the 1790 to present database

ccl/138/$ OR ccl/277/$ OR ccl/285/$ OR ccl/403/$

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