I think I might have stumbled onto a partial solution - I found
this listing on a South-African auction site of that is alleged to be a shell casing for the "6cm S-Bts K L/21", but it obviously isn't 60x64mmR. Instead it looks identical to those posted in this topic, so that got me thinking this might be from a colonial piece.
Sure enough, I found several references to a "
6cm Gebirgskanone L/15 in Kolonial-Lafette M.1902" manufactured by Krupp, including a reference in a
Krupp catalogue from the 1902 Düsseldorf exhibition. That particular album can't be found online, but, luckily for me, there is a report in French on that exhibition published in
"Revue D'Artillerie" which is available on Gallica, including a drawing of our mysterious piece.

As can be seen from the drawing, this is a modern piece with vertical sliding-wedge breech, possibly a derivation of the Gruson system, since Grusonwerke had been acquired by Krupp in 1892. The article also mentions the fact it used fixed ammunition (unlike the 6 cm S-Bts K L/21 whose ammunition was semi-fixed and which had a Krupp style horizontal sliding-wedge breech), but, unfortunately there are no actual pictures of said ammunition for a conclusive identification.