Each pair is numbered in the ‘under’ between the blades. “matching marks (symbols or numbers) are sometimes seen near the pivot area inside both blades. In Sheffield, these were hardened and ground together. In some cases these marks indicated that the two blades were forged together from the same piece of steel to ensure that both were of the same hardness. This coding enabled them to be matched together after they were tempered, ground and polished. If one blade was even slightly harder than the other, it would wear down and blunt the softer blade.” The Cutting Edge – Antique Scissors, Carolyn Meacham 2006
These days, with increased control over the alloy content of steel and the precisely controlled industrial heat treatment of the parts, this matching isn’t the issue that it once was. Obviously, given the complexity of having every item in batch being different, numbering the halves is still important in the process.