I have one with my tap and die set but my slide hammer is at work so I couldn't check. I just googled and found out one manufacture that actually tells you what the size and thread is for THEIR 5 lb but mentioned it MAY fit with other attachments so I just guessed.
I took a long look at a 5/8 "x18 bolt at the store to joggle my memory. I'd say it's about the same size and the only other thread size they had for that was an 11, and I'm sure the threads weren't that big. Anyhow, the tool is made and I think it's the right size and will work good. Even cut in some grooves on the duck bill to hold the studs. Should work out fine.
On another note, I just saw dent pro has a new 110v studless stud gun for $650!!!! Apparently, it's just like their 220v that's around $1000 but probably not as powerful. If I were on the market for a stud gun I'd just get that!