I don't think there would be a locknut. Mine has the roll pin (top picture).
There was an assumption before that the pin was later replaced with the adjustable post, but it seems from this thread that it was the other way round: earlier models had the adjustable eccentric? Do the later bikes, say circa 2004... still have the roll pin or was there a change again? ~ Not that it makes any difference ~
As regards its purpose, Baldini wrote a coupla yrs ago: "I think the pin is a stop for pawl arm. Guess that w/o it, even against spring pressure, the pawl arm may come away too far from shift wheel pins & allow uncontrolled movement of wheel."
The position of the pin makes no difference to ease of parts assembly (Docc).
I think it's clear from all the previous discussion and trials and errors and then fixes, that THE thing that causes the spring fatigue and break is the mismatch of spring coil and boss sizes, which causes obvious binding, stress and then *snap*!