Very few UK resellers, I can only recall two or three, commissioned special "own label" movements from Waltham, this grade 641 from 1918/19 looks to be a P.S. Bartlett with the addition of a double roller. The total production run of this grade was 150 open faced movements and 350 hunter movements.
It is signed "Preston Senior, Especially made for Prestons Ltd, Bolton, By Waltham Watch Co, Waltham U.S.A.".
I have also seen 1908-635's branded for Thos Russell of Liverpool and a lower grade, probably a 15J branded the "Preston Junior". A couple of others I know are now with a collector in Australia.