What is this cutting tool used for?

Question

It appears that one smacks one side with a hammer and the other side cuts something, I suspect that its used to cut the lead jacket on really old underground telephone cables, it's little over 1/8" thick, about 6" long.

Answer 1

That's a cable sheath splitting knife. They are designed to split insulated wires. They are intended for low voltage applications, such as with many telephone wires/cables.

That particular one appears to be made by Klein Tools, model 1515-S:

https://www.kleintools.com/catalog/cable-insulation-knives/cable-sheath-splitting-knife

Here is the full datasheet for it:

https://data.kleintools.com/sites/all/product_assets/documents/brochures/klein/kleintools_datasheet_1515s.pdf

Here is what it looked like when it was new:
image source: Klein Tools

They also come in different shapes. For example, here is one made for Bell system cables (model 1515-1):

https://www.kleintools.com/catalog/cable-insulation-knives/bell-system-cable-sheath-splitting-knife