Can I tell what Generation IPod I have by the Model number

I have what I believe to be a 8GB 1st Generation IPod Touch.
The Model number as reported on the System Preferences/General/About page is PA623LL
I see some people say they have an PA623LL/A or PA623LL/B Model numbers.
Can anyone tell me what the Model numbers are for the various generations of the 8GB models are?