Maybe you can confirm if 18.5 mount is metal or plastic......it seems I am the only one that claims that it is plastic.....and also the barrel?
The mount is definitely plastic, it was too easy to scratch it with a scriber to be steel. The barrel might be a metal alloy, I didn't dare to try to scratch it, it's still in pristine condition...
To get an all metal series 1 lens, you'd have to get the 32mm/1.2, a verrrry nice lens, but still overpriced (seen prices up to 700€/840 US$, paid mine 400€ for a boxed sample).
The 18.5mm is like a 50mm in FF terms (there is a 2.7x factor between sensors), and the 32mm is like an 85mm (86.4mm actually)
One nice feature of the 18.5mm is the rather inobtrusive shade (original: HB-N104), similar to the one that equipped the 45mm GN. The original is rather expensive (up to 50€, almost half the price of the lens!), a compatible clone is a more affordable 8€.
Ciao from Massimo