Waterman's  numbering system was developed to be  a reliable reference to distinguish models, their overlays and the size of their nibs. 

The early numbering system Waterman adopted till 1917 was however rather different from what we have grown used to make reference to. The basic reference to the size of the units column remains unchanged while the digit in the tens column to indicate a safety pen was originally #1 instead of #4 (#1 was also used to identify eyedropper fillers). 

Generally, in the early system, safety pens were better identified by  the suffix "s", in order to distinguish them from the simpler eyedropper fillers with fixed nib. 

In most cases the mechanism of very early safeties  makes the nib protrude out of the pen spiralling around its axis, while later versions have a straight movement.

This Waterman 16s in plain black ebonite is an early specimen in excellent condition. It has very fresh imprints and does not show any signs of oxidisation.

The mechanism is straight and not spiralling.

The beautiful Nr. 6 nib is very smooth and nicely flexible.