The problem here is that you are looking for one currency. And that you are looking for currency.
With the society you describe, there is little pressure towards it. There is almost no trade beyond the community level, and the existing trade will be mostly local.
At community level you actually don't need much currency; rural communities in "developed" countries like England and France in the 16th, 16th and 18th centuries would sheldom use money except for "imported goods" and taxes (and a part of taxes would be in locally produced goods). You remember who took three cabbages from you yesterday, so you go today to his house and ask for half a chicken. Playing the system (failing to pay debts, for example, or claiming debts that don't exist) is very hard in small, closed communities.
Additionally, such small communities won't have much to trade. Survival will take most of their efforts, and whatever surplus they can get is probably not very different from town to town.
Historically currency1 developed because:
* A way of certifying the quality of the bullion in the currency.
* A show of sovereignty; the first thing new kings (and claimants to the throne) would do was to mint money with their name/likeness.
Lacking a strong authority able to back it2, there are many hurdles to any proposal. In some areas your proposal may be so abundant as to be meaninless, in others too scarce. If there are big differences between areas (as your example of a tribe settling in Fort Knox) the people in worse off areas may refuse it (why give the effort of a year of work for what, to someone else, is just getting into the vault and getting an ingot?3).
With that said, an unified currency is not a necessity. You may have different currencies in different areas, with some interchange rates that are variable depending of the situation.
A significnt alternative for local trade could be credit based in an honor system; if a trader went to town 1 to sell A but there was nothing interesting to him there, a person of authority there could buy A in exchange of a letter (or simply a verbal agreement!) so the trader could go to town 2 and get B from another local authority (who could then ask payment in whatever town 1 could offer).
And, last but not least, the need of survival would make a currency based system very dangerous. 130
So you have a good harvest and get your boxes filled to the rim with (let's say) floppy disks. Good, you are the top dog of your village. But next year there is a drought and everybody harvests are poor. Since floppy disks are not edible, nobody gives you a single carrot for all of your savings. Since there is no honor/community system protecting, you are left to starve.
Now, in the town next door, they do not use currency but they store the food surplus. When the surplus is extra (you have your stores full with last year harvest, and this year harvest is near and will be plentiful) you may exchange some of it for other goods. If the harvest is bad, you just eat your savings.
1 Please be aware that I am talking about currency and not about bullion (e.g. raw slumps of gold and silver).
2 And that needs currency in order to manage their taxes and budgets, because it is the only significant group of people working to grow their own food.
3 And now you are thinking "But the people in Fort Knox may keep it a secret". Well, no, because they will begin using their gold instead of farming/producing gooods and people around will know that there is something fishy, even if nobody talks about the vault. 130