I personally put together the surviving ex-Reading equipment list mostly from input from RCT&HS members and the internet. It's been a somewhat fluid list that I update when I come across new information or when what I've posted has been proven to be false through reliable sources. As far as the number of GP-30's still in service, I don't know whether the correct number is four or seven. Feel free though to contact me with any updates on surviving equipment. I'm always looking.
Mike, RCT&HS Webmaster