Perhaps I should have spelled out my reasoning in detail. I do so now.
Grandson John B REID, aged 9 in 1881, and James B REID (son of John W REID & Maggie Ann BROWN), aged 3 in 1891, could not possibly be one and the same.
In 1872 (grandson John B REID's approximate year of birth) John W REID (b 1861 as we know) would have been about 11 years old, and Maggie Ann BROWN (born 1867, as Margaret Ann BROWN, daughter of Melville REID & Agnes ANDERSON) would have been only about 5 years old!!
In the light of known ages, the only serious contenders for parent of grandson John B REID (b ca 1872, St Andrews) would be Helen's daughters Christian BLACK or Helen DRUMMOND.
The IGI does not have an appropriate John REID nor does it have an appropriate John DRUMMOND nor even a John WARDLAW.
What it does have is a John BLACK, b 26 Mar 1872, St Andrews & St Leonards, mother Christina BLACK, no father named.
JAP