Wikipedia
A plate number is a number printed in the margin of a sheet or roll of postage stamps which shows the printing plate used to print the stamps.
A plate number is the serial number of a printing plate. It is printed in the selvage or border of a pane of postage stamps.
Philatelists and stamp collectors analyze or collect plate numbers and stamps with different plate numbers. The American Plate Number Single Society (APBSS) is an example of a specialist philatelic society which focuses on plate numbers. APNSS is affiliate #178 of the American Philatelic Society
Usage examples of "plate number".
There would be, I knew, an immediate radio alert, but I had no fear of road blocks or any such form of molestation: once the licence plate number was received at HQ I'd be left alone.
He copied it down, accessed the motor vehicle department and got her license plate number and description of the car.
If I had a license plate number, she could probably pull an address.
She dug in her other pocket for a pen and jotted down the station wagon's license-plate number.
He gave him the license plate number and added, Hey, let it go, Landry.
O'Hallinan spooled the form out of her typewriter and noticed the Tahoe's plate number scrawled at the bottom of her notebook page.
Perhaps someone had jotted down his license plate number and was even now being congratulated by the police.