Thanks to all for the replies.
I did purchase the safe, it's an 8x10, and is in very good condition, no dents, good finish, etc. It stays open at the top, needs just a bit of encouragement again about half way down, and then closes on its own from there. I tried coaxing a little fine grease with a toothpick into where the sides of the roll top panels disappear into the track, and that may have helped some, but not a lot.
I think the answer would be to turn the safe upside down with the rolltop open, and then spray or drop some kind of liquid oil right into the track, so that it gets up into the radius between the top of the safe and the front where the rolltop has to turn the corner. I hesitate to do this, however, for concern that the lubricant might somehow migrate and contaminate the paper.
It's not really such a problem to just close it each time, and I'm already quite into the habit of doing that with my paper boxes up till now. I'll try it for a while and see if it bugs me to the point where I'm bold enough to squirt oil into it.