This gadget was used to groove the hot top to accommodate the 120 quarter-wavelength radial ground wires that surround the base of each tower. Most modern AM towers are series-fed and insulated at the base. These are no exception. If you've ever seen the specially made toroidal transformers that supply power to the beacons on AM towers without shorting out the base insulator, you know that you can't mount ordinary lighting fixtures on series-fed AM towers. Everything has to withstand quite high voltages because the towers routinely take lightning strikes.
Protection is afforded by a ball-gap across each tower's base insulator.
The gap is adjusted to fire at a voltage just barely higher than that which appears across the base insulator on modulation peaks. The landlord obviously didn't know all this. The consulting engineers must have taught the landlord his first lesson about AM towers. So the landlord removed the lights from the towers, and after the office building was up, he bit the bullet and put in lighting standards to illuminate the parking lot. Back came the grooving machine.