Complex sentence: There is though a difference, in that 1960s, was the era of closed shop, and migrant labour often had to join unions.A possible explanation is that in 1960s strong unions prevented exploitation of migrant labour and this stopped immigration lowering wages.
Split into simpler sentences: There is though a difference, in that 1960s, was the era of closed shop, and migrant labour often had to join unions. It could be that strong unions prevented exploitation of migrant labour and this stopped immigration lowering wages.