Parker 51. Not a bogus Demo, not the well more common all-clear single jewel 1946-8 pen, but a First Year Demonstrator. Backdrop is David and Mark Shepherd's Parker 51 book, page 98 or so.
I've probably owned 8 of the all-clear later Demos. Have seen couple dozen, not to mention all the fakes. Probably though have seen but 2-3 of this sort, though no doubt more are out there.
regards
david i
http://www.vacumania.com