BS 3602 Steel Pipes and Tubes for Pressure Purposes
Part 1 Seamless and ERW, Part 2 Sub Arc Welded
Don't know what the 35 relates to as the notation used in BS 3602 describes the manufacturing process and minimum yield stress e.g. HFS360, Hot Finished Seamless 360n/mm2 Minimum Yield Stress.
ASTM A106 grades are rough equivalents although the BS does go into more details re the high temperature properties.
