Post 1918, Berlin was divided into policing districts each with a Police Regt. Each had a different elaborate cypher on the front, topped with a crown and flanked by the these long Bandeau. They are quite distinctive, and different from the wavy "W" Wappen of the Preußen königlichen Polizeiverwaltung in den Provinzen. I only have a photo of the 4th Berlin Police regt with a slightly different center cypher. This appears to one of the other 3 Berlin Police regts?
I would have to disagree that this is a studio Prop. Being in the military, there is just no way a soldier would be photographed in a studio with a helmet that was not his own. It's like me getting my photo taken holding an Infantry Beret. Not going to happen. Post war, the Reichswehr and Police and everyone else it seems used M1915 Bluse pattern uniforms. I am going to vote that although this looks like a Grenadier Regt, it is actually Berlin Police, and that they just share common characteristics such as a cypher on the strap and Litzen on the collar.