What's the name of this NPN configuration?
Here is no name for this transistor configuration. It simply boosts your current driving capabilities for the 50Ohm load and inverts the logic level from your optocoupler. 
While this configuration might work, there is a more efficient way of controlling the load. In your circuit, when the optocoupler is not conducting, the second transistor always dissipates energy. 



In this picture, you use the current driving capabilities of the optocoupler directly. Depending on the CTR (Current Transfer Ratio) of the optocoupler and your driving current of the optocoupler LED, you need a resistor (R2) to limit the current through Q1. The resistor has to be small enough to drive Q1 into saturation. 

Short example:

A transistor goes into saturation when both the base-emitter and base-collector junctions are forward biased e.g when the collector voltage drops below the threshold of the base-collector voltage (about 0.4V-0.6V). 
The saturation voltage (collector-emitter-voltage) of a common NPN transistor like BC546 is about 0.2V. The 50Ohm load will in this case be driven with a current of (5V-0.2V)/50Ohm =96mA. The BC546 has a DC-Gain of minimal 100. From here we can determine the needed base-current to (96mA / 100) = 960uA. So you need at least 1mA base-current to drive Q1 into saturation. 
The normal Current Transfer Ratio of most optocoupler lies in the range of 30% to 200%. If we assume a CTR of 50% then you need to drive the LED with about 2mA to get Q1 into saturation. R1 is only for protection, so you can increase it's value depending on the collector-emitter-voltage of the optocoupler to limit the maximum current.