I kept saying we only have two photos of the string quartet in which our family cello Heinrich and his eponymous owner participated, but I was wrong, I've rediscovered a third. To avoid confusion, I'll reunite the three here (they also appear in my flickr album "family history").
This is the one I rediscovered most recently,
this is the classic group portrait, dated April 1927 (UPDATE 20.5.2023 - I've now replaced my ancient photo of the photo with a much better quality scan),
... and here is the quartet in action.
I only realised last year that, zooming in on the sheet music in the last picture, one can read that they are playing BRAHMS. He only published three string quartets, so that narrows it down dramatically.
All photos were taken by my grandfather Richard, who was the only child of the cellist and a keen photographer.
I'm still none the wiser re. who the other three musicians were, see my search appeal here.