It looks like (take with grain of salt) they used references to PLC addresses as the basis for some of the tags.
I would look in the PLC-5 file for B190:915 as a first guess.
Often, RIO communications to old panelviews used block transfers to allow for data outside the assigned I/O data block to be used. Search the PLC-5 for BTW and BTR instructions that match the rack and group of the Panelview.
If the old Panelview is shutdown, you will likely find errors on those block transfers when monitoring online. I don't believe that the newer Panelviews support RIO. The last one I had to work on almost ten years ago was an older Panelview plus with RIO adapter and it was sort of a pain. If you are not using anything tricky like ASCII tags and a handful of other things that don't translate easily, then it should not be too painful to get it all working.
I think you are saying that you intend to connect a DH+ channel from the PLC-5 to the Panelview. This should make things much easier. The block transfers can go away, and you won't be restricted to the same addresses the old HMI used. Those restrictions can be a blessing in the fact that they force you to be organized and communication efficiency is mandated by the design, but I digress.
Back on point: Search the PLC-5 file for BTR and BTW instructions with the same rack/group/slot assignments as the old HMI. That should lead you to the tags that are not part of the I/O block. it is entirely possible that there are no block transfers being used, but only the "I" and "O" addresses assigned to the old HMI. Some small applications don't need more than a rack or fraction of one.
Another side note: I once worked on an application with hundreds of tags on the screens, but only 14 of them in the tag list. The genius (and this guy really was good) that designed it used the screen number as a pointer and had the PLC code "multiplex" all the tag data depending on which page was displayed on the PV550. This was great for him and for the old SLC that didn't support BTR and BTW instructions, but a pain for me to figure out how to set limits on a data entry point for just one item.
In the new DH+ unit, any tags that are in that I/O block will probably have to be moved to new addresses and the corresponding PLC logic may need to change accordingly. I could be wrong on this last point. The PV+ may let you write to those addresses, but I am not sure if the PLC-5 will put up with it. My PLC-5 and RIO experiences are old, dusty and rusty.