Try using the "UPLOAD" button at the bottom of the input screen, rather than the "Add image" button at the top.
You have a pretty valuable and pretty special pen. In your last post does it say "512 1/2 VS"? That would be a solid gold overlay, #2 nib, slim, vest-pocket (short), safety pen, dating from approximately 1908-1918. I think the style of the nob on the end would further narrow it down to 1910-1918. I thought it looked like a safety when you first posted the picture, but the model number pins it down.
Have you, or can you, turned the nob at the end of the barrel? It should cause the nib to retract into the barrel, which is how you would cap the pen, and how you would fill it (by pouring ink right over the nib into the barrel). Generally you do want to follow the advice about ink that Shangas offered, but Safety's are actually more tolerant of tough ink like India ink. Because the nib and feed actually soak in the ink when the pen is capped and are not exposed to the open air, they are much less likely to clog. Still, better to be safe than sorry.
Here is some information on Waterman Safety pens -
Waterman Safety Article at Vintagepens.com And on retractable safety pens in general -
Safety Pens.
Also, this would have been an original Waterman factory overlay (there are some custom jewlers overlays that are put on Waterman pens), since it has the 5 in the front of the model number.
I am really not a Waterman expert. If David Nishimura does not chime in here, I would suggest contacting him - see Vintagepens.com, where the information above comes from, or PM him, username "vintagepens". He might also be a good person to contact if you want to sell the pen - I think he would offer a fair price and is a well connected dealer (and I have no affiliation with him, just respect his opinion and knowledge highly).
I have no idea of the value of this pen, but suspect it is at least in the low-to-mid three-digit range.
Great pen. It would be a shame to sell it, but life happens, I guess.
John