I think SRs have titanium pivots while NRs have steel, so there whould be some weight savings with the SR.
BTW, I thought that none of the Supervitus framed Peugeots ever came with Campy SR gruppos. Sounds like your PSN might have been modded up with the SR gruppo at one time.
Chombi
84 Peugeot PSV
85 Vitus Plus Carbone 7