Q:

GPL version not specified, and actual license text not provided with the source

I'm working on porting a very old program (1994) that was released under the GPL. However, the only reference to that license is this text in the header of the C files:

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation.  You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

Should I then assume that it's version 1, and can I release it under a later version of the GPL?

A:

From the text of the license:

If the Program does not specify a version number of
  the license, you may choose any version ever published by the Free Software
  Foundation.

This clause is in every version of the GPL.

