Thanks, Damon. “Stored number” is exactly what I want from your description as well as my own.
I think I’ve found it. Creating a “customer number” by using the dropdowns to get number-only image numbers and stored number sequence numbers, then swapping the order so sequence # first.
Now I just have to take a few more shots and download them to see if it works. What I already have can stay as-is. There aren’t any duplicates and adding the sequence number will make all the new ones different anyway.