A Slag Glass Lamp with Roman Theme

One of my favorites, this one has been in my family as long as I can remember, but I just recently came into possession of. I'm not really sure of the manufacturer and I didn't see any markings when I looked, but I'm also not exactly sure where they'd be located.

I have seen a similar lamp posted here, so I believe the base is correct. I am, however, missing the heat cap for the top of it and it needs a cleaning after sitting in an attic for a few years.

That said, I'm not exactly sure how to go about properly cleaning it (what, exactly, is a "mild" soap?) and I don't want to mess it up.

Anyone out there have any input on the make or pointers on how to effectively clean it without hurting the value?

Thanks for looking!

EDIT: I'm not sure how to rotate the images, but I'm doing this from my phone. My apologies for them being sideways.