Belt drive to chain? Help.

Apparently, (and I'm going to measure to be sure) trek decided on 132.5mm rear spacing to give flexibility in hub options. So rather than stretch the alloy frame I'm planning on using 130mm and spacers. 130mm fixed wheels seem readily available, where as 135mm seems to be hubs supplied only. Where does 130mm come from? Hybrids?

Alloy or alu?