OFTP2: valid characters in Partner Name (in Windows)

I've noticed that if I use certain characters (oddly, something as simple as a hyphen ['-', ASCII 45]), Mendelson allows them in the name, but when creating the resulting directory in the messages folder, it silently changes this to an underscore.

Before I spend a bunch of time on trial and error with this, can anyone tell us what characters other than a-zA-Z0-9 and space would be allowed by Mendelson and *not* converted to an underscore?

I understand that it's a cross-platform product so you probably wouldn't want to trouble with different character rules for each platform, but wondering if a simple answer could be given without my having to resort to the source code.