I'm having trouble understanding the arguments in http://arxiv.org/abs/hep-th/9510209
They propose that M-theory on the orbifold R^9 \times S^1 \times S^1 / Z_2(1) yields Type I' on R^9 \times S^1, wheras M-theory on R^9 \times S^1 / Z_2 \times S^1(2) is E_8 \times E_8 on R^9 \times S^1. What...