This is maybe what you need. The caption for this is " 1st Cameronians using Vermorel sprayer in HQ trench, Bois Grenier Sector, 20th May 1915."
The operator has clearly adopted 'the combat stance, by numbers, one' for the photographer whilst his embarassed oppo stands by thinking 'what is he doing now'
I had one of these until I disposed of most of my 'hardware' about two years ago. Mine was in all respects identical except it was missing the transfer (but had the embossed lettering 'Eclair Vermorel' as in the photo) and had been lovingly buffed to a perfect shine by a 'helpful' previous owner. The tank was made of copper. Even the original red rubber hose and correct nozzle were present.
It was the sort of thing most other collectors were underwhelmed by. But I was really pleased to find it as I specialised in gas equipment (mainly masks)
It should be possible to obtain one in a brocante in France. Mine came from the little show at Sailly-Saissel about 12 or so yearrs ago. I repainted the tank in a matt sevice green as per the IWM version which I recall was not by Vermorel.
I wouldn't know if these sprayers were obtained from UK manufacturers, but this type of crop sprayer was pretty common at the time. The fact that the name Vermorel stuck and was used all through the period of service most likely means that the initial purchases were from that Company. My advice would be to find an original Vermorel, but clearly as the IWM example shows others were used.
As an aside for the uniform collectors among us..... look at the dark almost black shirt of the man holding the tank ( I've had a couple of these otherwise normal issue except for the colour ), the detail of his braces and his natty neck scarf. Also note the (probably red) rings around the top of his issue? socks. I've not noticed this before..but towels etc were marked with a red stripe. Both are wearing the early 'face-mask' style respirators, no doubt because of the concentration of gas they would have to deal with at trench bottom level. Both are also wearing cap comforters.
These 'sprayers' look very familiar, back in the 1960s (my early teens) in Ireland I used to see these used by farmers for spraying anything from weed-killer to potato blight killer.
They were copper and had a pump action lever on the right as I remember, with the brass spray arm connected on the left by a red rubber tube. They were filled by removing an insert copper lid on the top and inside there was some sort of a pressure chamber through which the liquid was gravity fed before the pressure of the pump action forced it out the spray arm and nozzle. The liquid came out in a fine spray and on a windy day it was necessary to cover your nose and mouth. The one I was familiar with was very old and the webbing straps were in tatters and with a full load cut into your shoulders. Also when full if you stumbled walking down the line of the potato drills, the liquid would slosh about and unbalance you then spill out the sides of the lid (not a tight fit) and onto your back and shoulders.
There was an embossed 'logo' also on the front of it but I cannot remember what it said but have an inkling it was French.
It was locally called a "BUDGET", where that name came from I do not know
Boy! but that sure brought back memories, I can still recall the smell of the 'bluestone' (copper-sulphate and washing soda mixed, now classed as a health hazard) sprayed on the potatoes.
They could be used also to disinfect an area or room e.g. Farmyard, stable so therefore a trench, latrine area etc.
You can still get plastic versions in your gardening/hardware store.