The difference is plating on the contact area.
PCN13-50S-2.54DS(71) : 2μm tin copper over 1μm nickel
PCN13-50S-2.54DS(81) : 2μm tin over 1μm nickel
PCN13-50S-2.54DS(71) is not available for sales,
as it is custom made specification for a specific company.
PCN13-50S-2.54DS(81) is available for sales.