The upper turret on the Ju 188E was B1 Stand ('B1 position') - 13-mm MG 131 in EDL 131 Drehturm (rotating turret).
(Btw, nose gun position was A-stand, upper rear of canopy B2, and lower gondola rear C-stand)
Pics of Ju 88B (Ju 188 design prototype) positions, from 'Bombers of the Luftwaffe' by Joachim Dressel and Manfred Griehl.
1. Ju 88 BV27, with Ju 188A and E armament - MG 151 in nose (A-Stand), MG 131s in B1 and B2 Standen.
2. BV 27 conversion test for Ju 188E and E-1 series
3. MG 151Z test fit in B1 stand (for additional details)
4. MG 151 (B1 Stand), MG 131 in B2 stand.
5. Weal Ju 188E cutaway, from 'Deutsche Kampfflugzeuge des Zweiten Weltkriegs' published by Karl Müller Verlag.