Toroidal ferrite core size

[ThomsCircuit]

Wanting to build a project and one of the parts is a 1/2" toroidal ferrite core. I want to get the correct one so I'm asking if the 1/2" is OD or ID.
I have the wire and a slick tutorial on how to wind one. Also, if I could substitute the 4401 with 3904.

Thank you.

Here is the project.

10 Year LED Flasher​

IC1: CD4001 (cmos quad NOR gates)
Q1: 2N4401 (NPN transistor)
C1: 100nF (0.1uF) ceramic capacitor
C2: 1nF (0.001uF) ceramic capacitor
C3: 10uF x 12v tantalum capacitor
R1: 4M7 resistor
R2: 2M2 resistor
R3: 4k7 resistor
LED: ultra-high efficiency LED (available current is very low)
T: 1/2" toroidal ferrite core & 2 meters (6 ft) of 24AWG (0.5 mm) enameled wire
BAT: 1.5 alkaline AA cell. (battery holder optional)

The instructions to wind the transformer are as follows

 

[augustinetez]

I'm asking if the 1/2" is OD or ID.

Outside.

Also, if I could substitute the 4401 with 3904

Probably. I say that based on the guys comment of the low current draw.