It's quite all right. The next play-through, simply use them and replace the treasure map with a conterfeit one, then while you're going through rusty, near a skeleton (if I recall corectly) you'll see the golden shimmer. The item is there.
Don't feel bad, on my first few play-throughs I didn't do more than 84 quests, and for quite a while, I didn't know I could get the submarine in Tekron underground by activating it, rather than paying 30k for it. I used to pay for it all the time, since I had plenty of cash at that point.
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