Perhaps you need to look at Shenandoah County VA - near the Augusta Co border at the time....1780s or so.
An Abraham Bird of "Red Banks" on the North Branch of Shenandoah River - near Hawkinstown, had a son born in 1760s who married a Hannah Allen.My Turney family had adjoining land to Abraham Bird and his Hawkins son-in-law in this area.
I found a pension records from NC on an Allen in Rev War who said he was born 1741 in Maryland, when 21 years old he moved to Frederick Co VA and lived there 7 years and then moved Sept 1770 to North Carolina.That would put him in Frederick Co VA about 1762-1769.
Let me explain something......all this area began as Frederick Co VA, later Augusta Co got the southern part.Where my Turneys lived went from Frederick to Augusta....then the line was moved and a new county was formed named Dunmore.It was Dunmore a short time and then changed to Shenandoah.Later Augusta was cut down with other counties formed.So depending on the time - this Red Banks fell in Frederick, Augusta, Dunmore, Shenandoah.......just so I don't confuse you.Rockingham County would have been Augusta in early times - and earlier still it would all have been Frederick.
Just thought perhaps since we know an Allen family was in the Red Banks area on North branch of Shenandoah....and connected to Bird family, that might give you a clue.Some of the Birds went to KY, some on to MO.My Turneys went to Eastern TN, while others went about 1787-1789 to KY.
I know there were Allens in other parts of VA - over the mountains in the eastern part.....;but you are correct that some were in the Valley of Virginia area in your time frame.