Just a thought;
GM probably made a few hundred thousand or so of these coolant pumps over the years. And, (as you know) I am no expert but I would imagine that they were designed to be very flexible in regards to application. A centrifugal pump doesn't care how it is mounted as long as the shaft is parallel to the longitudinal axis.
Anyway this pump was most likely spec'ed for many different engines (i.e. marine, locomotive, emergengy generators, etc.). It could be that the upper weep hole is actually the lower weep hole if the pump was intended to be mounted upside down as compared to a SBC.
BTW - I paint everything either black or silver.