I have also received a very helpful email from Erica Johnson, Genealogy and Local History Researcher at the Ewart Library in Dumfries who says that she is unaware of any reliable sources for the following three baptisms which appear in some on-line family trees:
James Ker 12 APR 1718, Dumfries. [It appears from his burial in Mouswald in 1789 that he was born c1718.]
John Kerr 2 JAN 1756, Mouswald Banks, son of James Ker and Martha Moffatt
Rachael Kerr 16 JUN 1762, Collin, Mouswald, daughter of James Ker and Martha Moffatt
She says that "The Mouswald Old Parish Registers have a note on the front which says that there were no regular records kept at the Kirk until the minister took office there in 1765, the entries in the register before 1765 are copied in by his hand from “all that is left” of the records he could find when he took office." Best wishes, Nic