A hydrometer is used to measure the specific gravity of liquids, which is the ratio of the density of the liquid to the density of water. An alcoholometer is a special type of hydrometer which uses specfic gravity for estimating the alcoholic strength of beverages such as beer and wine.
In the late 19th century, hydrometers were required to have special revenue stamps attached to the case to show the required tax had been paid.
Here is an 1877 hydrometer seal I recently won on eBay
. It looks to have a small bit of malt syrup stain on it.
If you have some of these stamps, please post them.