Whenever a product was too good to discard but not good enough to meet the factory's first quality standard, the backstamp was scratched or marked and then either given to staff or sold in staff or retail stores at a discounted price, usually 50% or even more.
The method of marking the underside to indicate a second was varied. It would have depended on what stage the flaw was discovered. The backstamp transfer could be partially cut or pinholed. Common flaws of seconds were; mis-alignment of graphic transfer, size variation, glazing or gilding flaw, a minor impurity or firing pitting, colour variation etc. Please note that gilders often put a test mark on the back/bottom....it does not indicate a flaw, it was simply used to test the gilding or sometimes to identify the production team.
Do not believe a claim that a second quality is as good as a first quality, the seller is just admitting that they cannot see the flaw. The item became a second for a specific reason. It may have been minor but it was enough for the manufacturer not to risk their reputation. You would be surprised just how many seconds were manufactured of this pattern.
Don't be afraid to ask questions such as...
Can you explain in more detail why you have described this item as mint condition and what is your return policy if we do not agree with your claim?
Is this an actual photo of the item and it's backstamp?
Notice how the rose has been defaced.
Source for text and one image
Source for another image
Source for first two images: me