Are you still looking for the birth of John, s/o John and Frances? I have the Montgomeryshire Records No. MR/PR/58, Parish of Llanfyllin, Register of Baptism & Burials, 1752-1787.
Page 38, #938:
1775: born, 15 Nov, John, s/o John Bryan flax-dresser and Frances
baptized: 15 Nov