John Brown Shipyard and Ross and Duncan Shipbuilders

[AsH62]

Hi.
My grandfather was demobbed to John Brown on the Clyde at the end of WW1 according to my dad. There is no reason to doubt this except that we have found a letter giving the details of his apprenticeship dates from Ross and Duncan at the Whitfield Works, Govan. It gives the dates of his apprenticeship from 22nd Feb 1915-22nd May 1920. During this time he was at Chatham Docks in the Navy as an Artificer but that isn't mentioned. The only logical conclusion is that he was apprenticed to Ross & Duncan but part of the apprenticeship was in other places.
This is a conclusion reached with no facts at all, please can any one tell me if that is possible. Dad is 86 but still very sharp and I can't imagine that he would muddle where his father went after the war.
Thank you
Alison