I thought I should post an update about the results of the work that Mr. McWilliams did for me. Two weeks after sending my check to the Pennsylvania Archives, I received a package in the mail with all of the references that David Lindsay appeared in (5), in the Militial Loan Accounts for Westmoreland County. Mr. McWilliams was correct in his guess that David Lindsay's signature would appear in the Distribution Vouchers. I now have a copy of an original signature of David Lindsay.
I have learned (once again) that it pays to never give up when researching an ancestor. For me, the third try was the lucky charm!