Buck, I agree with everything you say, and I have been considering these same things. I have also held and studied the rifles you mention, and although it is speculative without a signed example, there are enough similarities of these to Joseph Angstadt's work that make me wonder if Adam Angstadt made them, since he and his son Joseph worked together apparently in one shop. In other words is there a possibility that the unsigned pieces you mention are Adam's (not the Abraham / Schuylkill County gunsmith but Adam born around 1740) or were made perhaps by both Adam, and his son Joseph, prior to Adam's death around 1812. From the records I have a sense that Joseph was much more reliant on working with his father Adam than other independent gunsmiths - although again speculative.
My own feeling as for the "A. Angstadt" guns is to agree with those who believe these were made by Abraham Angstadt, born in 1783. Based on the records, I found Abraham located up by Orwigsburg (Berks County until 1811 then became part of Schuylkill County) already by 1810. Maybe he spent part of his time training with a gunsmith in that area and decided to stay there.
Similar to Eric's post, has anyone ever actually seen a gun signed "Adam Angstadt" and not just "A. Angstadt"?