Does anybody know how to tell the difference between the 1913 waterlow 2/6 (SG 399/4000, Scott
173) & the 1915 De La Rue 2/6 (SG 405-408, Scott 173a).

There is a more expert collector of these stamps than
I am, on this NG, but I'll have a go....

DeLaRue stamps generally have smaller perforation holes
than the other printers, and have one larger hole at the top
of each vertical side. The Gum appears more 'yellowish'
and is a little patchy in appearance - also the colour of the
stamp can usually be seen through the back of the paper!

Hope this may help,

David.