[ChrisMAg2]

IMO, it is not the sight for a MG 151. AFAIK no Ju 88 (the part nr) carried a MG 151 in a movable gunmount with an elevated sight like this.
In the center square you can read the word "Bola". This would indicate it as a sight for the "Bodenlafette" or the so called C-stand or ventral position. Usualy a gun for this elevated sight would either be a MG 15 or a MG 81(Z). And the plane typ could be something like a Ju 88 A-5 with a Bola 39D VE (MG 15) or a Bola 81Z (MG 81Z).
You can read and see this i.e. in Squadron-Signal 085, Junkers Ju 88, pt.1