The soft master known as a lacquer would then be electroplated with a metal, commonly a nickel alloy. This and all subsequent metal copies were known as matrices. When this metal was removed from the lacquer (master), it would be a negative master since it was a negative copy of the lacquer. (In the UK, this was called the master; note the difference from soft master/lacquer disc above). In the earliest days the negative master was used as a mold to press records sold to the public, but as demand for mass production of records grew, another step was added to the process.
The metal master was then electroplated to create metal positive matrices, or "mothers". From these positives, stampers (negative) would be formed. The stampers would be used in hydraulic presses to mould the LP discs. The advantages of this system over the earlier more direct system included ability to make a large number of records quickly by using multiple stampers. Also, more records could be produced from each master since molds would eventually wear out.
Since the master was the unique source of the positive, made to produce the stampers, it was considered a library item. Accordingly, copy positives, required to replace worn positives, were made from unused early stampers. These were known as copy shells and were the physical equivalent of the first positive.
The "pedigree" of any music record can be traced through the stamper/positive identities used, by reading the lettering found on the record run-out area.
Music Records@ Vinyl-Record.co.uk