I have been informed over on Photrio that it was "Arista Premium" that was rebranded Kodak. Maybe my memory of that is off. I'll check later tonight when I'm downstairs where the film fridge is. There was a brief time around 2009 that Kodak was selling film through Freestyle rebranded, like Ilford and Harman had done before and Foma does now, apparently when they thought film was dying and they had over stock. By the time I heard Plus-X was discontinued I couldn't find it but I could find the Arista version. I shot some back in the day and it's definitely Plus-X.
I expect the stuff I bought then, about ten years ago, refrigerated since, will be fine. Not so sure about stuff in the 90s. I bought that Delta 400 but then moved to Georgia. When I got back into 4x5 I just went with stuff I could still buy and didn't crack the box of stuff that I knew I couldn't get any more of. Right now I'm in the midst of building out more which will include a "real" darkroom with running water, not the room with holding baths I used back then, which will help a lot and mainly what got me interested in getting back to photography (well a few other stars aligned too, as it were.) Once I do I'll dig all the film out and see what I have and decide.
HP5+ is a nominal ISO 400 film but like a lot of 400 films excellent, arguably better, results can be had by giving a stop more exposure so it's hard to say if yours has lost any speed or not, but it does look just fine to me here.

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