Is that correct?
Yes, go to the library and get a copy of the Scott US Specilized Catalog. It does not have to be a current year. Try to get one from 2021 or before as that has the stamp identifier in the front.
Or buy one on line as cheaply as possible, say 2016 or earlier. heck I have a 2004 I give you for free at the cost of postage. But that would need to be approved by a moderator.
Then check the threads on SCF about stock sheets, stock books and holders for envelopes. For a good overview (not to purchase there) search for Lighthouse Publication in the USA (New Jersey I think). The have a great assortment shown on their website. Most can be had for less elsewhere especially if you don't mind pre owned.
You do have a solid collection of stuff and I think you may have fun "working it up" as the saying goes while to arrange the material in safe containers.
Yes were are all just custodians of the material we own until it is passed on to the next custodian.