We've been experimenting with the 651.
I have a piece that I wrote on with red, blue, black and green a couple of weeks ago.
It leaves a ghost image if you just wipe with your finger, but still wipes off clean with a damp paper towel. Red is definitely the most difficult to wipe off.
We'll see after another couple of weeks if it's still wiping off clean.
Thanks for the Fellers tip. They just opened a new store in my area, may have to give them a call.