Under a 30x and 60x loupe it looks to be 512 but the number is fairly worn as is the lettering underneath at the "6 o'clock" area. I will look again.
edit - Upon looking again it does indeed seem to be 512. The first and third number, a 5 and 2, are fairly obvious. Have to look really hard for the impression of the top half of the 1 inbetween them. The I only say it is a 1 because of the serif at the bottom of the number and what appears to be a stright line upwards instead of going in the direction the top half of the 2 would go. And it does not curve at the bottom like a 5. Also, originally when I looked at the pen when I received it and started posting about it I thought the mark at dead center between the number and PSF at the bottom is indeed a 1/2 which I originally thought was a Waterman "W".
I also noticed something I never saw before. A hint of red inside the impressions of the numbers?
Edited by Ray Cornett, 15 March 2014 - 13:02.