Arch McClain was a trustee when the town of Mount Sterling was incorporated in Montgomery County. E.g., Montgomery Co. Deed Book 3, p. 336, contains a deed dated 11 March 1805 beginning "To all to whom these presents shall come we James Ward Joseph Simpson Micajah McClurg and Arch-d McClain trustees of Mount Sterling send Greating Know ye that by virtue of an act of assembly establishing said town 640 acres of land was vested in trustees and established a town by name of Mount Sterling and by virtue of an act entitled an act concerning towns in this Commonwealth whereby the said trustees or any three of them are authorised to make conveyances to purchasers under certain restrictions Now This Indenture witnesseth that the above mentioned trustees in order to carry the said acts into execution and for and in consideration of a Bond given by Nathaniel Logan to James Pogue ..." (Other deeds on pages 338, 395, 396, and 401 of the same book begin with a nearly identical section. In Jan 1806, on pages 597-599, there are several deeds from the trustees, recorded by a different clerk, where Archibald's name is spelled "McIlvain".)

It appears that the family name was originally McIlvain, and some descendants kept that spelling, while others (including my own branch) apparently gave in to the frequent confusion with the more common McClain by adopting that spelling, and a third group emphasized the distinction by spelling their name McGilvin or McKelvin.

1810 census of Montgomery Co., Ky, p. 384A, line 15, recorded 5 Aug, lists in Mount Sterling:
Archibald Mcilvain, 3 m 16-25, 1 m 26-44, no females, no slaves, 3 “other free persons, except Indians, not taxed”

The next line is for James Mcilvain, 1 m under 10, 1 m 16-25, 1 f < 10, 2 f 16-26, 2 slaves.

I have found no record of Archibald’s wife or children. Tentative dentification of Archibald as the father of Charles McClain who married Comfort Sexton is based solely on the fact that Charles and Comfort named their first son Archibald, and it isn’t a common name. Inclusion of Beverly McClain as another son is based on family tradition that Charles and Beverly were brothers.

It may well be that they are both grandsons rather than sons, though, especially considering that the 1810 census shows no males under 16 in Archibald’s household. (Charles should be about 15, and Beverley about 3 or 4.)