re. 3rd MAG on Merkava 4, there appeared a pic of some new Merk 4 modification, which has roof add-on armor re-configured so that loader's hatch can be opened, so I guess it was in order to mount another MG
(Or it may be just case as with M109 Doher - our modeller's magasine once noted that as for accessories etc., anything goes because Israeli crews are very creative in what to mount on their 109's and no two pieces have the same kit
)