Need Help With Identifying A Marking On The Back Of A Stamp

Hi everyone,
I would appreciate your help with identifying a marking on the back of a German stamps.
On the reverse, there is a red handwritten signature (see attached images). The seller on ebay described it as an "expertizing backstamp by German Phil Soc (Philatelic Society)."
I'm a bit skeptical, as the marking looks more like a handwritten name than a typical expert mark.
My questions:
Does this look like a genuine expertizing mark to you?
Is there any known German expert or society that used this type of signature?
Or is it more likely just a collector's or dealer's mark?
Any insights or references would be greatly appreciated.
Thanks in advance!

I'm far from an expert, but an expertizer's mark running upward on a diagonal from the bottom left corner is a mark for an unoverprinted stamp, according to the BPP "code." On an overprinted stamp, the mark in that location/orientation would be inverted.

Someone who knows what they're talking about can chime in. I could be all wet.

ETA: This was posted a few years back: