HP never manufactured tubes. They, as many, produced lots of equipment that used tubes and hence distributed relabeled tubes to their customers who assumed that they MUST use HP tubes in their HP equipment The relabeling saga begins.
I've seen RCAs relabeled as HPs and probably others that I'm not remembering. If the "5751" marking is etched in the glass inside a rectangle (actually, a "flattened" octagon) near the top of the tube, they're indeed RCAs. If not, a photo would help a lot.