Kept because condition matters more than rarity in almost every album I
have opened, and nobody says so plainly.
Perforation gauge
Holes counted over twenty millimetres. The same design printed with a
different gauge is a different stamp entirely, and the difference is
invisible until measured.
Watermark
Thinning in the paper, visible held against black with a drop of
fluid. Some issues differ only here, which makes an unremarkable sheet
worth checking twice.
Hinge remnant
The paper scar left by mounting. Once it is there the gum is never
original again, and a hinged copy sits a full grade below an unmounted
one no matter how clean the face looks.
Cancellation
A light strike leaving the design readable is wanted; a heavy one is
not. On cover, with the envelope intact, the postmark can be worth
more than the stamp it cancels.