Here is another WW1 shell case that I have and is believed to be exactly the same size as the one in Post 1. This one was purchased with several other shell cases by my late father from an antique shop in the late 1970's.
Although I don't tend to buy "trench art" I can appreciate an item if it is an original, period, piece that commemorates an event, such as a battle. I think that this one has been rather nicely done.
This one is very lightly, finely, hand engraved with a design that includes the name "Peronne" which is on the Somme and is where a number of battles were fought. I believe that the Australian army fought a major battle here during WW1.