A German private and a Feldwebel

More than a year after my last post, Thursday’s Rui’s Renditions is back… on a Friday
A German private and a Feldwebel from an unknown unit posing with British ordinance and shell casings at La Bassee, France, 25 October, 1915. On the back the sender wrote:
“Werter Herr Köhler, Sende Ihnen etwas neues aus dem Westen, dies ist ein Englischer Blindgänger hier in Labassee.
Herzlichen Gruß aus dem Westen sendet Ihnen Ihr Wilhelm Hannemann”
(Esteemed Mr. Köhler, I’m sending you something new from the West. This is an English dud here in La Bassee. Best regards from the West from your Wilhelm Hannemann)
On what I believe to be the shell from a BL 15-inch siege howitzer it is written “English dud” and “John Bull sends his regards”.
Only twelve BL 15-inch siege howitzers were built by order of the First Lord of the Admiralty Winston Churchill, having been operated throughout the war by detachments from the Royal Marine Artillery (1).
In my opinion, the 12,8 cm (5 inch) shell case could be from either a BL 5-inch field howitzer or a BL 60-pdr field gun, and the 8,5 cm (3.3 inch) from a QF 18-pdr field gun.
Notes on the Feldwebel (see b&w original in comments): Given the lack of reflectiveness, the tressel seems to be of the new subdued type instead of the usual silver or gold. Waffenfarbe chosen was red for infantry but the grey is of a medium tone so there is a lot of room for interpretation. The same goes for the state Kokarde: Is it strong light reflexion or is that a different colour in the centre other than Prussian black? If not black, then the only possible matches that make sense to me are green (for either Sachsen-Meiningen, or Sachsen-Coburg und Gotha, or Sachsen-Altenbug), or dark blue for Schwarzburg-Rudolstadt. Given my indecision, I kept it black but I’m not entirely happy with it.
(1) “British Artillery 1914-19, Heavy Artillery”, Dale Clark, New Vanguard
Original card property of Drakegoodman