It can't be.
I was on the same computer, using the same OS (XP)
One day it produces underscores, the next day it made spaces. Maybe i dreamed it?
Now that is something that even i don't understand. I hope it stays that way.
But on 98 it always makes underscores.
Therefore in conclusion, i say that on Windows NT/2000/XP ALT-255 produces a space " ", and in Windows 95/98/ME it produces an underscore "_"
But as you say Mac, we can always depend ALT-0160 and your CallURL program, and not on Windows