Prior to WWII was the great depression many Canadians went to the United Kingdom to find work and joined the Canadian Army after it arrived overseas, there were at least a half dozen in the 48th and they all had the U prefix.
Maybe you should send for the man's records or go the Access to Information route. I was checking out a chap who was an American and was killed while serving as a Canloan Officer his attestation paper indicated that he was to go to the 48th and the book about the Canloan Officers said that he was a 48th Highlander but he was with the 48th Holding Company for about two weeks then was posted to CMHQ for inteligence work.
Clyde Law's book "Regimental Numbers of the Canadian Army" says the U prefix was for men enlisted overseas. The number that you give would indicate that the man was enlisted in the Royal Canadian Ordinance Corps and the numbers for that unit started at started a U 600.