The Royal Navy 38-ton gun (shown here in land-based version along with a 7-pounder mountain gun) was a 12-inch gun, similar to the guns aboard HMS <i>Thunderer</i>. It was described in the January 9, 1875 edition of <i>Scientific American</i> along with this illustration, taken from <i>The Engineer</i>.
Unrecognized signature (lower right)
<i>Scientific American</i>, January 9, 1875