My experience with Schwalbe tires in ETRTO406 (Marathon, Big Apple, Big Apple Plus, Big Ben Plus, MotoX, Pick-up, Marathon Racer folding, Durano, Durano Plus, Ultremo, Pro-One) is that they all were always a little wider than their nominal width.

Marathon, Marathon Plus 35x349 for Brompton exact width.

Only the Big Apple 50x355 for the Birdy is slightly narrower than 50mm.