sett (n.)

see set (n.1).
The extra t is an arbitrary addition in various technical senses, from a lawn-tennis to a granite set. Each class of persons has doubtless added it to distinguish the special sense that means most to it from all others ; but so many are the special senses that the distinction is now no more distinctive than an Esq. after a man's name, & all would do well to discard it. [Fowler]