Engraving an Elysee Parthenon

Hi all, hope this question isn't out of place here, it's the only
place I could find that looked helpful.

I have just bought an Elysee Parthenon red laque fountain pen for my
wife, she lost hers a few years ago. I was really looking for the
Trajan Rouge but couldn't find one.

On the cap, quarter way around from the clip, there is a gold 'patch'
which looks like it could be engraved - can it? If so, what metal
colour is underneath and how deep should the engraving be?

I hope this question isn't as sacrilegious as asking for cola in a 25
year old Scotch.

Thanks,
Rick Fox
(Manchester, UK)