Comment from W1TP Tom Pereira: "That is a submarine (under the water) telegraph cable key used to put positive and negative voltages on underwater telegraph cables. The positive voltages are for dots and the negative voltages are for dashes. This dual polarity helps to improve the ability of long cables to carry dots and dashes". It looks very much like the one made by Bunnell in America."
Comment from K1BH Bill Holly: I have to concur with Bill Burns on the "B" and "L" (Bill Burns http://atlantic-cable.com suggested that "B" was for Battery and "L" for line) and also believe the key must be a Bunnell though i would guess it's a slightly earlier vintage than mine as the binding posts appear to be a older style and my keying arms are slightly longer allowing the keying knobs to extend beyond the base by about 3/4 of their diameter.