then it is a Northern Pacific rattlesnake (Crotalus oreganus oreganus)..The field guide you are using is quite a bit outdated if it is still referring to these animals as "western rattlesnakes"...The whole viridis complex was split up quite some time ago by taxonomists...
A note about re-releasing animals back into the wild...PLEASE do not do so if you've kept this animal more than a few days...you may be unwittingly introducing pathogens into wild populations of animals that have no resistance to them...Also, consider that in many areas it is illegal to re-release animals that have been in captivity longer than 72 hours...Please check your local & state regs before doing so.
If you do release, please do not release this animal very far from where it was captured. Many studies have shown that North American crotalids spend virtually their entire lives in a set geographical area...removal from this area often leads to the death of the snake. When i release animals that have been found in people's yards, I try to keep the release site within a quarter mile of where the snake was found...
-AzAtrox