1881 Census, 3 Glassary, Argyll. Lachlan Bell, 38, born Lochgair; wife May, born Garve; Mary J J, 8; William A, 6; Lachlan, 4; John D, 2. Mary, William and Lachlan born Minard.
From the IGI at FamilySearch, Mary Jessie Jones Bell was born 17 November 1872 at Glassary and William Archibald Bell on 11 September 1874. Unfortunately the IGI stops at 1874.
Scotland's People index has both births, Mary registered in Kilmichael Glassary and William in Cumlodden. It also has Lachlin Bell, born in Roseneath, Dunbartonshire, in 1876, and John David Bell, born in Lochgilphead in 1878.
So I think the reason you can't find Lachlan b 1876 may be that he was registered as Lachlin?
Now, from the 1881 census Lachlan senior said he was 38, which would mean he was born in 1842/1843. He says he was born in Glassary. However he was obviously a bit muddled about which of his children was born where, so maybe his recollection of where and when he himself was born wasn't quite accurate.
Have you got a copy of the marriage certificate of Lachlan Bell and May Fraser? If so, you need to take a look at that to confirm the names of his parents. They were married in Glasgow in 1872.
The index at Scotland's People doesn't list a Lachlan (any spelling) to Archibald Bell and Mary McTavish. It does list Flora Ann, born to parents Archibald Bell and Mary McTavish, in Glassary in 1820, and in the same parish Jannet, parents Archibald Bell and Nanny McTavish, in 1828.
You obviously have the 1841 census at Knock Lochgair, Glassary, showing Archd Bell, 40; Mary, 40; Alexander, 15; John, 12; Jannet, 10; Hugh, 8; Peggy, 6; Duncan, 4; and Lachlan, 2. Apart from Jannet, none of these children are in the baptism index at Scotland's People, so the likelihood is that the records of their baptisms have not survived.
Similarly there may in fact be no surviving records of the marriage of Archibald Bell and Mary McTavish.