Vol II of the Holtzapffel work (page 676 et seq) describes by what methods the firm maintained the sizes of their taps and dies. Master sizing taps were made, by which master dies were made. These master dies were then used to make sets of secondary master taps for use in the workshops to make the dies for forming those for sale in the store. When secondary masters were all worn the first masters still existed, having only been used once. In this way for the first time it was possible for ultimate users to buy taps and dies many years after their original purchase which still maintained the original sizes at least within workable limits. Although such firms as Maudslay and Roberts had organised similar systems previously for use in their own works, Holtzapffel's were the first to supply repeatable screw tackle commercially and it was not until Whitworth (between 1841 and 1858) produced his system for sale that another such facility existed. Even after Whitworth's system became general in engineering, certain firms for special purposes kept up with the Holtzapffel threads; one firm of printing machine makers even used them still in 1950. So far as is known, none of the Holtzapffel masters has ever come on the market and been recognised. Thus this set represents an important step in the history of precision metal work for the student or museum of the history of screw threads. The set is unused; presumably there are two each of the smaller sizes in case of breakage. From the fact of its being unused it must be one of the secondary sets. Whether the original first masters will ever come to light is doubtful, though to be hoped for.