OK, that confuses things! HMS Vernon was the Torpedo and mine establishment. Was it a support site for divers? The only other thing that comes to mind is a Foxer.That is a large clanging device that was towed about for distracting acoustic mines and torpedos. Though this is fixed to the lake/sea bed.(Is the Rosie for autograph cokkie that keeps appearing at the tyop of the page a futher attempt to distract?)
You said no to minesweeping and de-gaussing ships so why else, except for ilegal fishing do you have electric cables under water? working a chain ferry?
Cleared a lot of obvious then. A 2 ton sinker? Must be quite a beast. A difrent era. De gausing kit for ships? Or the opposite to map a ships magnetic feild?
Yes, but a bit up the wrong track, a submarine detector is the next guess. :-D I have seen something similar, which is why I thought torpedo. The RE had a lot of weird river and costal defence gadgets in the late 1800 early 1900. So how close am I?
Trench Art is a rather dubious feild. As early as 1919 locals were collecting stuff and engraving it for tourist curious. There are modern examples even now, enough dated casings etc still lying about.