Let's assume, for the moment, a child born out of wedlock, father's name Munro, mother's name Ross, in Bonar. If the mother was born in 1891 and married in 1917 then the feasible dates for such a birth are perhaps 1907-1917, though probably slightly narrower. There are thirteen births with the surname "Ross" and four Munros between 1908 and 1916. To narrow it down a gender and a first name would assist, if you have them.