Great find! Be careful though, that $5 gun could end up being one of the more expensive guns you own.Don't ask me why I say that.
What modifications are you planning?
The allen screw in the breech is .050" but I use a T05 torx which has served me well while many people that use allen's strip them out...The only other real difference from the phase 2 is that in around '01 they switched to an aluminum valve, and the exhaust valve has a delrin seal instead of the brass one with molded in green stuff.
Nice results.