[Calm And Relaxed Voice]
Don't worry, my nose isn't bent out of shape, and I hope not to get you your nose bent out of shape, but what you have stated only works if the mac hasn't been modified at some point in it's life.
This thing may be only months from it's tenth birthday, a decade's a long time to no have any mods at all.
None of us know if it is the same as the day it shipped, or not. It might be, but for all we know, this may be the case of a single core 1.6Ghz with the motherboard, CPU's and PSU from a quad core (with a badly damaged case and fried GPU) and a random processor cover and graphics that were just sat on a shelf by the work bench. We do not know, because we cannot know.
The serial number would likely be from the case, but internals could be changed.
I've never done this to a G5, but if I had a reason to, I wouldn't hesitate.
So I say again, the only way you can actually be 100% sure beyond any doubt, is to take a look at what is physically under that cover.
[/Calm And Relaxed Voice]