I don't have reliable information about a specific work titled "Subtle Souvenir" by Mark Tobey in the etching medium to provide you with an accurate description. While Mark Tobey was a renowned American modernist painter known for his distinctive "white writing" technique and abstract works, I cannot verify the details of this particular etching without access to authoritative sources. I'd recommend consulting museum collections, art databases, or Tobey's catalogues raisonnés for accurate information about this work.