Is "air flow meter" the same as "mass air flow sensor"?
 

I can't find the MAS at FastLane

Yes, it is the same.

Your 91 wouldn't have a Mass Airflow Sensor (more properly abbreviated the MAF, this is what is used in MB technical literature). It has CIS-E injection, which uses an Air Flow Sensor. They both, in a loose sense, do similar jobs. The Air Flow Sensor is a plate that is in the air flow path, and the air flow deflects the plate, which is attached to an arm, which moves a potentiometer.
The Mass Airflow Sensor is a bit more complex, it uses the flow of air to "cool off" a heated wire or piece of film matrix, which causes a change in the sensor reading to determine the "mass" of air being drawn into the engine (mass in this case being the combination of total cubic feet per minute, temperature and humidity of the incoming air).
Hope this isn't WAY more info than you needed.

Gilly

ps The air flow sensor on the CIS-E system is commonly referred to as the "air flow meter", as you have found on the online catalog.