For clarification of the conversation, here is the Scott listing for #635 fromthe 2020 catalog:

So we are not talking about a changeling here.
To be honest, both the original and reprint varieties of this stamp are cheap and most dealers (and
ebay sellers) will not take the time to differentiate as it makes no money for them to do so. Additionally, many
ebay sellers have only one to sell and dont have comparison examples, etc.
This is where the collector has to invest their own time. Looking at
ebay, most of the plate blocks and plate singles available are #21185 and 21186. I would start by obtaining one and build from there. Used dated examples shoud be readily available in dealer $1 boxes for reference purposes.
Add: to use as a reference, the Kansas/Nebraska overprints will be the earlier shade of #635