Best Venue
Given the historical significance of the officer and the specialized nature of the archive, this should be consigned to a premier militaria auction house (e. g., Heritage Auctions, Cowan's, or Morphy Auctions). Prior to consignment, document the 'rock solid provenance' in writing and take detailed macro photographs of the flag's fabric, stitching, and hoist to confirm its mid-century construction.
Upside Potential
If the 'rock solid provenance' includes direct photographic evidence of del Valle with this specific flag, or official military retirement transfer paperwork, the archive could exceed the high estimate and approach the $3,000+ tier at a specialized militaria auction.
While the signed documents appear consistent with period typewriters and authentic signatures, high-ranking general officer flags are frequently reproduced. Without close-up photos of the flag's material (wool bunting), stitching (appliqué stars), and hoist hardware, the flag cannot be definitively authenticated as a mid-century original.