The periscope is not directly related to rear guns, as the Ar 234 had almost nil view to the rear it was required to spot enemy aircraft closing-in from the rear.
And please do not assume an installation in production aircraft if this system was tested in a prototype - lots of weapon kits were only found in protos but were never used in prod a/c.
In 1944 it was not uncommon to produce the basic aircraft in assembly plants and to fly it to equipment centers to install special bomb, recon or other equipment for its designated combat role. So the assembly plant could concentrate on churning out as many a/c as possible while the equipment centers were used to fit special stuff.