Sleeper

Sleeper - a stamp or other collectible item that seems to be underpriced and may have good investment potential.
Slogan - the brief sentence incorporated in modern machine postmarks/cancellations, such as 'Post Early for Xmas'.

Sleeper - Stamp or other collectible item that may be underpriced, potentially being a good investment.
Sleeve - A flat transparent holder use to protect and/or store covers.

Pup: British word for sleeper, a stamp in a dealer's inventory that is worth more than charged.
Purifie au (city): purified at (city); to indicate mail had gone through disinfection process.

See also: Inscription, Pair, Die, Used, Unused