This is really a very good Watercolour box of an early date. The pans are not held by the little spring clips that Winsor & Newton patented in the 1880s so this box predates that time and in my opinion based on experience over a very long time I suspect that this box could be ten or twenty years older maybe more. 
It is of their earliest designs where it is encountered in the Royal Letters Boxes which celebrate their patent granted in the 1840s. 
So it is of a good size and built very well indeed. 
I often talk about these early boxes in glowing terms and the reason is quality. 
On the face of it a steel box that gets wet a lot is a concept possibly flawed? The obvious reason is that it will rust away and many later ones have done exactly that. However with these early ones they matched the paint that covers both inner and outer surfaces with its requirements and that of course is to stand up to a constant wetting. 
As I am a museums conservator I have had to restore some of these things in the past for clients and of course proper restoration is using the same materials or at least nearly so that were used in their construction. So I took it onto myself to identify the the paint used when originally made. I believe that they used an oil based varnish paint which though can be tricky to use gives a very good protection against wet situations. It won’t of course stand up very well to other solvents but typically they are not used in painting with Watercolours. 
So essentially what does this mean. Well in my opinion if you want a very well built box that actually will last if looked after then buy an early one. 
This particular example is in full working condition at least as much so as the day it was made it contains 19 of its original pans stamped WN and all 30 are ceramic so they’ll last forever too!
When the paint of which there is still plenty here is used up simply fill with tubed paints. 
It of course has some cosmetic issues but they mean nothing if they’re not likely to get worse and if looked after this box will out live us all!
As I say it is of a good size and both comfortable in the studio or outdoors. The things it must have seen. 
Approximately 18 x 9 x2 cm