Sometimes, You Don't Notice The Stamp!
The hobby of collecting postmarks is known as Marcophily. It's a fascinating area, with specialist areas within the hobby. Avid postmark collectors are quite willing and able to look past deficiencies in the quality of the stamp if the postmark is scarce enough!
Postage stamps were introduced in 1840, and the first postmarks were designed to obliterate the stamp to ensure that it couldn't be re-used. The first postmark in wide use was this one, a Maltese Cross. Many different types exist, and this one can be identified as an Edinburgh Maltese Cross
