Convert millivolts from shunt on DMM on a 50A, 75 mV Shunt
If 75mv is the full scale reading for 50A, then :

Edit 2 : So

(50A / 75mV) * 1.5mV = 1A


Where 50A is max shunt current, and 75mV is full scale reading and 1.5mV is the reading obtained by the OP.
Note, one can see that the mV, as units, will cancel leaving the result to be A.

Edit : the shunt has already been calibrated, you may find small cuts in it to "match the resistance to give 75mV at 50A.