I recently bought several of these for my old Centurion, which runs a 6-speed freewheel. When I received the chains however, I noticed that on the front of the package was printed "Non-Index." It never even occurred to me that there would exist such a thing as a non-index chain, and now I have three of them. The only information a cursory googling turned up is this old thread, which describes the difference in form, but not in function.
My question is, does it really make a difference? Will something terrible happen if I use them with an indexed shifter? Should I return them and buy different chains?