SK socket marking ???

[wtn1271]

Got this SK socket at an estate sale recently. It's a 9/16 deep (semi deep?), 12 point, 3/8 drive marked S•K 42418 USA N+ 9/16.

What does the N+ stand for?

I called SK customer service and they did not know either, (emailed them the pictures and spoke with someone that has been there for 12 years).

Any thoughts?

 

[Snaparxon]

When Stanley bought Proto I believe they marked their sockets with a sideways "S" to signify that it was a socket they produced most likely for warranty purposes. Perhaps the N+ is the same thing for SK?