I guess we still need a "tie-breaker" to verify if Koffel's B1 heads use 0.325" or 0.350" offset for those corner bolt holes.

My rough measurement supports 0.325", but my method could have some error.
With the calipers and dowel pins threaded in the holes, with a lock nut to square it to the threads, I came up with these other dimensions:
9.596" (9.600" print)
4.046" (4.050" print)

If I didn't just square up the vice on my Bridgeport milling machine, I would use the DRO for a better set of numbers. But it needs to be out of the way to put the head on the milling table. And I am behind on several projects that need my time.