Re: colt DA 38
RAC is Rinaldo A. Carr, who inspected the gun originally. JTT is John Taliaferro Thompson*, who inspected the gun after it was rebuilt in 1903.
*Yes, the Thompson of the Thompson submachinegun, who at that time was a captain and inspector of revolvers and machineguns at Springfield Armory. Whether he actually did the inspecting of that revolver or not, it was done, and marked, by his authority.