Heavy duty, higher test rings. Spec MIL-C5651B, Type II 1080HD used on: Piper-PA-16,18,18-“95″18-A, 20 (1952 & up),22,25. 1280HD used on: Piper J-3 SN827 up, PA- 11,12,14,17,18 (S/N18-3013),18-“135”,18