So in summary, quick answers AFAIK to your questions on what the displays mean:
Indications (in flight around 30 min after takeoff):
M BUS E
28.0 VOLTS 28.0
Your Main (M) Bus voltage is 28.0 V.
Your Essential (E) Bus voltage is 28.0 V.
M BATT S
+1.5 AMPS +0.3
Your Main (M) battery is slightly charging at 1.5 A.
Your Standby (S) battery is slightly charging at 0.3 A.
Yes, +Amps means the alternator is charging the battery.
Note 1): -Amps i.e. a negative value, would mean the battery is discharging, which could occur if you have various switches/equipment turned on and if for example (a) the engine is running at low/idle RPM such that the current generated from the alternator is less than the total current drawn, or (b) the alternator is not running at all such as when either the engine is stopped or the alternator drive belt has broken.
Note 2): The Main battery and Standby battery are both (nominal) 24 V batteries connected to a (nominal) 28 V system so I don't believe it is abnormal to have a slight charging current 30 minutes into a flight, but you could mention your observation to your aircraft mechanic to check whether those values are within the normal range.
Note 3): Those relatively low currents may not have caused a noticeable deflection in an old analog gauge. For example if that gauge had a range -40 A to +40 A then 1.5 A would have been less than 2% of the full-scale deflection so it could have appeared as if the needle is sitting "in the middle" at "0" even though there is 0.3 A or 1.5 A charging current.
That's just AFAIK but if the POH or a licensed A&P mechanic tells you differently then obviously go according to the higher authority!