You are right about the "E" taking the place of ohm or Ω. Maybe it is avoid imagined confusion. "R" is commonly described as the decimal point. Thus 4R7 = 4.7 Ω. Why the author of your link chose 470E instead of 470R or just 470 is a mystery.
In the circuit shown the Z80 is sourcing the drive current, 470R would be about right , give ~ 10 mA per pin , consider if 3 leds (or more) were all on... using a lower value resistor , may come close to maximum mA of the cpu. check the Z80 data sheet for absolute maximums.
You are right about the "E" taking the place of ohm or Ω. Maybe it is avoid imagined confusion. "R" is commonly described as the decimal point. Thus 4R7 = 4.7 Ω. Why the author of your link chose 470E instead of 470R or just 470 is a mystery.
No mystery about it, he's not a native English speaker (his email address is .DK), a number of countries on Continental Europe use E as a short cut for ohm instead of R as English speakers do.