Continuity mistake: When the Devil is a teacher in the classroom she is erasing homework assignments off the board. In the close up shots (and maybe 1 or 2 of the far away ones) the board's partially erased. In most of the far away shots however, the board isn't erased. (01:01:50)
Continuity mistake: When Elliot first walks to the Devil beside the snooker table you get a wide shot and you can see all the snooker balls on the table. The position and colour of the balls change in a later shot when the Devil takes a shot and pots all the balls. (00:09:50)
Answer: It definitely is a Montegrappa Oriental Zodiac pen, this brand has been making pens in Italy since 1912. I am not sure which of the three red pens in that collection (Oriental Zodiac) is the one seen in the movie, but given its association with the devil, my guess will be it is the Ox fountain pen.