There are still quantities of these available to dealers, so no markups right now. Your plan to keep them for decades is sound, however, not so many. The notes that were 50 cents in 1961 are today worth $5 to $20, depending on what they are. The Ruble note was worth only pennies in 1961, and perhaps a dollar, now. So appreciation has been good, but may be not going much higher. These were cheap in 1961, and still cheap today. Expecting common notes to become a rarity is not wise.
For a stash, however, 1000 notes may be a bit large. When it's time to sell, large quantities have to be jobbed out, and thus your returns will be lower. Might want to reduce that stash down to three or five hundred during the next decade, then keep the remainder, or distribute a bundle apiece to several heirs. Put single notes on
ebay and sell them yourself, putting the returns into a bank account, you can pass that along to your heirs in 50 years.
Do write out instructions to heirs as to what they are and what you expect to see happen with the notes. Put that with the notes.
I, too, have a stash of stuff I'm preserving for long-term appreciation, meaning decades. It's a mix of inexpensive material and better stuff, in quantities not to exceed 10 of anything, and generally less than 5 examples.