I have a clip of P/88 ammo and an 1890 Erfurt with what I believe is the original barrel (matching, not upgraded to 88/05 spec) as well as access to a machine shop and all the measuring equipment that goes with it. Let me know if you'd like me to measure anything to settle this debate and I will do so this upcoming weekend.
ETA: I also have samples of WWII German issue 7.92 ball. Unfortunately, I do not have any WWI-era spitzer ball to measure.