Replacement rubber gaskets are for female fire hose couplings or adapters. These gaskets will fit National Hose thread "NH", National Standard Thread "NST" or National Pipe Straight Hose "NPSH".
Applications

Female fire hose couplings
Female aluminum adapters
Female brass adapters
The most common fire hoses in the United States are 1 1/2" and 2 1/2". Some fire departments use 3" hose but they have 2 1/2" threaded couplings.