Yes, the older 340B is certainly more worn and in all likelihood was less consistent to start with. So, it delivers the same load a bit slower (don’t have notes in front of me to see exact chrono results). And that’s the most likely cause, or predominant cause of it failing to stabilize the 55grs.
Probably on that ragged edge. Both guns might have problems if you didn't live up high. That's why I run the calculator set at sea level. I live at 4900 feet.