It is also possible that this is a wedding portrait of Wilfred Washington Knowles who married in 1908 at age 20, and later married again in 1919 at age 31 and married again in the 1930s. However, there is a known portrait of Wilfred Washington Knowles c1908 and I don’t believe this is the same man.
Given it isn't William Wallace Knowles or Wilfred Washington Knowles, that leaves Robert Knowles as a possibility. Robert Knowles married Jessie Grayston on 18 June 1879 in Liverpool and this might be his wedding or pre-wedding portrait. His age at the time would have been about 27-28 which is in the estimated age of the man in the photo. See also Possibly Jane Grayston Knowles Portrait (c1879) portrait.
The Leonhardt family lent the original of this Studio Portrait to Dave Jordan in 1990 who photographed it. The Leonhardt connection to the Jordans is that Margaret Jane Jordan Graham and Muriel Leonhardt were Knowles sisters.
If you would like to add information about this picture, please contact Dave Jordan, a 3x g-grandson of William and Jane Knowles. The Picture Reference Number is: Digitized Negative: 161-07.