Patents.
3,911,598; 3,944,331; 3,944,332; 3,950,078; 3,951,519; 3,958,862; 3,961,843; 3,963,310; 3,963,312; 3,963,324; 3,967,881; 3,968,362; 3,973,830; 3,977,795; 3,978,427; 3,979,696; 3,982,253; 3,984,153; 3,986,769; 3,990,778; 3,992,681; 3,992,682; 3,992,683; 3,996,525; 3,996,528; 4,004,250; 4,009,933.