To be honest, if you're just trying to hold the value of your investment, you're probably better off looking at other investment opportunities.
Vintage banknotes, like virtually all other collectibles, are ultimately susceptible to market forces. A banknote today valued at say $100 has no guarantee that it'll still be worth the (buying power equivalent of) $100 in the future. It depends on whether anyone wants to buy it and what those collector's are ultimately willing to pay for it. If there's an economic depression, then fewer people will have the disposable income to spend on collectibles. If there's an economic boom, then you might get more collectors - but there's no guarantee they'll want to collect what you have!
If you invest in high quality US notes, that's not much good if the collector market is interested in, say, high quality Canadian notes.
There's every chance that the vintage banknote market could be a bubble burst, and then your investment could be substantially lower than what you paid - leaving you with either the option of holding it for longer in the hopes of recovery OR taking the loss. Of course, if you sold the right item at the right moment, you could also make a lot of money too.
But collectible bubbles do happen frequently. Look at the comic book bubble burst in the mid 90s, the beanie babies in the early 2000s, or more recently with cryptocurrencies & NFTs.
My point is that coins & banknotes are a relatively risky market, and if your interest is in stability rather than gambling for profit, I'd say you're taking undue risk.
If you're going to invest in things simply to retain value, you're probably better looking at Government backed things like bonds & gilts, or bond (or bond-like, e.g. ISAs) things from commercial banks. You'll probably still lose a little value as the rates they offer are rarely higher than the rate of interest [Though there can be exceptions!], but you can be fairly re-assured of some fairly comparable future value if they're operating in a fairly stable nation.
Of course, these are usually for fixed-term lengths so aren't particularly useful as liquid assets.
Or if you wish to invest in physical items, look at precious metal bullion (E.g. Silver or gold). I'd personally go for things that are purely designed as bullion rather than precious metal items - I.e. I'd buy silver bars rather than silver coins. Most dealers will only offer you the spot value. Coins are usually charged to you at a premium - assumedly for the artistic value. So, you need the spot value to spike more significantly to make your coins bought at a premium worth more than the future spot value.
Again, it's a speculative market but at least precious metals have an actual intrinsic value, compared to -essentially- a pretty piece of paper.
But at least bullion you can sell all day every day, so it's a fairly liquid asset. Buyers won't care whether it's bullion created by the UK's Royal Mint, or the US Mint. So long as it's provably bullion, they'll buy it.
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Anything you spend on your hobby of collecting you should be, mentally, prepared to write-off as a total loss [or at least, for coins/banknotes loss down to face value]. If you need to sell it, then any money recouped is simply a bonus.