Well, listed as a line level as it has become to many people through etc.,  but as a carpenter I submit here that mounting this level on a string would have been awkward at best, as can be seen in the pics the small diameter of the screw and the fact the screw itself is offset from the cast stops. A regular line level has little hooks to hang freely from the stringline. I would offer that the level was made to slip onto a framing square or ruler to act as an extender for more accuracy to use in place of a larger level to build lookouts or cornices for example, as there were not many offerings in the market like torpedo levels at the turn of the last century. A true antique, the bubble is still good.