Prussian: This is an Officers Kugelhelm (Ball top) for Fuss-artillerie (Foot Artillery) (=Heavy):
In 1872 the Prussian Artillery was split: Fuss-and Feld-artillerie:
this is Fuss-artillerie: with flat scales on the chinstrap.(Feld-artillerie is mounted troop: always convex scales on the chinstrap)
Officer: Officers stars on the ball-top base and officers Kokarden.
(the star at the front is missing...

)
Four possible Line- /Linien Regiments with this Wappen (plate) for the "Alte Regimenter" / "Old Regiments": With the F.W.R monogramm (Friedrich Wilhelm Rex). The other Line Regiments Wappen= the Line Eagle (Linien Adler) with the F.R. monogramm (Friedericus Rex).
Nr 1 Fuss-artillerie Regiment (Ostpreussisches)
Nr 2 Fuss-artillerie Regiment (1. Pommersches)
Nr 3 Fuss-artillerie Regiment (Brandenburgisches)
Nr 6 Fuss-artillerie Regiment (Schlesisches)
You should check the shell for "extra holes"...
Pitty about the missing Officers star and liner.
The top of the helmet shell looks a bit "sunken" and the shell is bit shrunk as well (rear spine). But: I like this Kugel Helm !
Rgds,
Francis