The Sakura Electric SE-2000 eraser
I would once again like to thank Artful Hinger for his pointer to the Sakura Electric SE-2000 eraser, to wit:
http://www.sakuraofamerica.com/electric-eraserAs to the wild scribbling on this postcard, a wise man once told me: "Never ask how they got that way."
After a light cleaning of the easy half:

Done! Q/ Any suggestions for that coffee stain? Bleach? No more eating & drinking during Stamp Time?

I am especially pleased to have revealed the 'instructions' block.

The art side of the card shows French Guyana's Kaw Channel, which seems to be a channel-in-a-river-thru-a-swamp-near-a-town-all-of-the-same-name(s).

The contemporary photo (below) is from one of these websites:
http://www.marais-kaw.com/ ... the Kaw Marsh Nature Reserve's own website
http://maguyane.over-blog.com/artic...9388686.html ... close-by contemporary photos
http://www.aspag.fr/2010/06/kaw-%C2...guisambourg/ ... kayaking opportunities
http://www.lefigaro.fr/voyages/2015...e-marais.php ... travel review
http://projetbabel.org/fluvial/rica_kaw-canal.htm ... illustrated by, would you believe, my postcard!

Cheers,
/s/ ikeyPikey