C W Johnston had three successes but not in the Senior.
1925 2nd in the Lightweight on a Cotton averaging 56.88 mph, 2nd in the Ultra Lightweight also on a Cotton averaging 52.08 mph and in 1926 1st in the Lightweight again on a Cotton averaging 60.20.
As for Fondu it appears he had five starts, four DNF and one place of 20th.
I went to the Isle of Man web site for the Fondu info as my 1978 TT magazine only details the first three places but you may wish to have a look there as there are some interesting facts there. Www.iomtt.com