Ken,
I think the H&A (single screw) Boxlocks were on the Forehand design. I don't believe the sideplated boxlocks were. I am more inclined to give credit to W.H. Davenport but have not made a solid connection other than he was H&A's superintendent during those years.

Back to the buttplates:
All my sideplated H&A guns have a plain buttplate with horizontal lines between the screws.
The H&A boxlocks all have the sunburst H&A plate.

John