I just looked at a box of those today and didn't notice a plus sign. They do come mixed with ProVs and Xs. I would suggest just going by the color of the number! That should indicate exactly what they are. They are practice balls for a reason, could have been a simple typo on the printing machine. 

Hey there, if I'm not mistaken, and someone please correct me if I am wrong. Titleist makes more types of golf balls then what's released to the public. They are usually for tour players only, I've seen a very popular Titleist player with ProV1x+. I've also seen another pro with a ProV1x with a dot after it. I have found a couple of these golf balls around the course where I work. I hang on to them because they are pretty cool. Didn't realize they sold them as practice balls, maybe overruns or something.

I heard someone say once that the + model has a slightly higher flight that tour pros prefer. I don't know all the ins and outs but that's what I got.