VERBS, MOODS OF:-- SUBJUNCTIVE: complete pastSubjunctive, complete present. (See Should for "if he should have.") The subjunctive with "have" is not very frequent. It is used where a past event is not indeed denied, but qualified conditionally, in an argumentative manner:
i.e. "If it should hereafter be proved that she have," "if so be that she have." So
“If, sir, perchance
She have restrain'd the riots of your followers,
'Tis on such ground . . . as clears her from all blame.
"Though it have" is somewhat similarly used to express a concession for the sake of argument, not a fact.
“If this young gentleman have done offence.
“For though it have holp madmen to their wits.