There is an interview in this month's (April) Military History magazine with an American veteran of the Spanish Civil War(Robert Steck). According to him, the unit was actually the Lincoln Battalion of the 15th International Brigade (the "Quince Brigada" memorialized in the song), which also had a Washington Battalion. There was also an English Battalion, but it is unclear in the article what Brigade they belonged to. The French volunteers were in the 12th International Brigade