Did you do that cooling modification to your B1 heads?
I am guessing this is an alternative way to do the same mod others have done between the exhaust ports. For water return to pump?

I wonder if this was a half finished modification. Since there is no way to connect a fitting, it is just a 0.75" dia hole. No threads, etc.

1993 Daytona, 5.50 at 130mph (1/8) 1.19 sixty ft (PG). Link to 572 B1 - Part 1