Definition of First of exchange
Where a set of bills of exchange is drawn in duplicate or triplicate, for greater safety in their transmission, all being of the same tenor, and the intention being that the acceptance and payment of any one of them (the first to arrive safely) shall cancel the others of the set, they are called individually the "first of exchange,” "second of exchange,” etc.
That's the definition of First of exchange in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.