Hi Peter, I've just dived into my storage box to find my Patterson safelight. Looks the same as yours and it has a 15watt Philips bayonet bulb. Of course, mine is for 230 V mains power so you'll need need an equivalent 110 v bulb for your mains power supply.
Edited: It's a bayonet fitting, NOT Edison screw. (But who knows; I think the US used more screw in bulbs than the bayonet bulbs used in UK, Australia and NZ.)
A curious aside. When I started out in darkroom work in high school, 1973, and bought my first setup the recommended colour of safelight was AMBER, not RED. The shop I bought from said that red safelights were no longer suitable and they sold me an amber bulb which replaced the ceiling bulb in the laundry that was my first darkroom. Over the next 40 years I replaced that bulb several times but I have never used a red safelight. I picked up the Patterson unit along with a similar red Photax safelight about 10 years ago in a yard sale.