It's a UK hardcover quarterly magazine (which is just about to become a quarterly anthology . . . which is really what it's been all along!). I think they must have e-mailed Charles a PDF copy of the issue.

Postscripts is distributed in this country, though not (yet) by a major publisher. You can find copies for sale online. Prod me in a few weeks, once books have had time to reach the US, and I'll try to offer a more helpful answer.