Wide Ratio 5 SpeedTrans centering Spring

This isn't a new issue and I don't reall plan to fix it, but I'm curious what causes it.  My '77-'78 5-speed trans does not center from the left.  The 5/R gate centering spring works fine, but it doesn't center from the 1/2 gate.  Is this a common issue and is it fixable without a major overhaul?  The trans only has about 70,000 miles on it.  I bought a rusty original owner low mileage '77 30 years ago to harvest the trans and interior which are both in my '78.  The car sat around for many years, and at some point, the centering spring quit working.  The trans shifts great other than the lack of centering.  It's the only Datsun trans I've ever had with this issue.