It seems that if you enter any device (e.g. AA gun) in a slot after a slot which contains a kind of radar (e.g. type 13) this device is treated as electronics device. (Can be seen on info screen in game.) Maybe this influences only the info in the detail screen, but maybe there is also an influence to the function of the device. In RHS this applies for example in the case of Yamato (503 and later upgrades) and Shinano (502). I suggest to use a radar device always as last device of a ship/aircraft/unit. This is the save way.
I think you may be correct. I think single 25mms were added by someone - to be correct - as an afterthought.