Just some additonal notes (that need confirmation):
The marriage between William and Agnes has been mentioned already in 1920. If there is any document that you should have in this collection it would be this one, to let you verify properly their parents' names. If you want to work further back on the Henderson/Roberts and Ross/McGregor lines, the parents' marriage or death certs should name their parents to let you work back with confidence further.
This looks likely entry for 1891 census:
William Henderson 22 tramway conductor b. Scotland
Kate Henderson 24 b. Ireland
William Henderson 1 b. Ireland
Mary A Henderson 3 b. Ireland
Address: 100 Bernard St, Glasgow/ Bridgeton
I can't see parents' marriage in the Scottish records of
www.scotlandspeople.gov.uk Maybe they married in Ireland.
1901 census:
William Henderson 34 Tramway Timekeeper b. Glasgow
Catherine Henderson 36 b. Ireland
William Henderson 11 b. Glasgow
Richard Henderson 9 b. Glasgow
Mary Henderson 2 b. Glasgow
Henry Gordon 40 boarder
Address: 118 Bernard St, Glasgow/ Bridgeton
William Jnr looks to have been so little when family moved to Glasgow, likely he saw himself as a Glasgow boy. There is a birth for a Richard Henderson in the Bridgeton area of Glasgow on SP. The original image would include the date and place of parents' marriage.
Monica