The idea is to convey the idea that it's capable of emulating System/360, System/370, System/390, and z/Series mainframes, while not stepping on any of IBM's trademarks. I think I've done it. It doesn't look too bad, even at 64x64 pixels:
It does lose any sort of identity of the individual characters when shrunk further, however. I've submitted it to the Hercules users' mailing list for their comments. Hopefully, they won't shoot it down. I think it's sufficiently different from anything IBM does that they won't have a complaint.