Thank you. It's an interesting piece. Describing it correctly presented a challenge. I understand these are common and of little value from reading other threads. Couldn't find any threads that discussed this particular oddity. I never considered it might be ink from another envelope. Makes total sense.
sheetguy2 has the correct description of this error. The relative common albino, an error, is treated as as a freak by collectors of US envelopes. The setoff by itself is also a freak. Being postally used is unusual but not uncommon and was covered in a message from the admin at POD hq.
You do have an unusual envelope with a value somewhere around 20-25% more than catalog value (with any downward adjustments for condition).