It definitely has all the characteristics of hexadecimal. The question marks are sort of spread around the first half of the characters and aren't in the beginning or end. I'm sure I could read everything clearly when I wrote it down so I put the question marks there intentionally. All characters used in hexadecimal are located in the string of characters, so I didn't intentionally leave a specific character or two out and just add those in place of the question marks. I stacked they string of characters in a way I couldn't tell if it was two 40 character keys or an 80 character key. If I only use the top line I have a 40 character string with no question marks, but that is an odd length.
The transaction to your address took place about half a year before I learned about Bitcoin and I don't know which wallets besides Bitcoin-GUI were common or popular. So, lets assume you used Bitcoin-GUI.
As far as I remember if you did export a private key of your receiving address then Bitcoin-GUI would've given you a private key in WIF format (not 100% sure about this as I didn't do such things with Bitcoin-GUI in 2011 when I first used it). What you say about your characters doesn't appear as a WIF key, so it might be that you converted it to hex for some reason.
Are you absolutely sure, you don't have any sort of documentation about your "backup"? If not, as a reminder to other readers, stay away from selfmade obfuscation or backup methods without documenting it properly. There's very likely lots of footguns with such approaches.
I don't quite get or can visualize what you mean by "
The question marks are sort of spread around the first half of the characters and aren't in the beginning or end." and together with "
I stacked they string of characters in a way I couldn't tell if it was two 40 character keys or an 80 character key. If I only use the top line I have a 40 character string with no question marks, but that is an odd length.".
A private key in hex is 64 characters. You have 66 if we ignore the purpose of the questions marks (could be a decoy). You say your hex string starts with "001", so what about if you drop the leading "00" which would give you remaining 64 hex chars and try to use that as a hex private key. Also try your 64 char hex string in reverse! Who knows what "crazy" scheme you invented? No pun intended, just being mildly creative...
I have some other idea if you would explain me in more details what you mean by "
The question marks are sort of spread around the first half of the characters and aren't in the beginning or end.".
From what I understood so far, the question marks don't replace any missing hex characters as you say you have all of them, 0...F, in the written characters already. 14 missing characters hidden by the question marks would be a bit too long to remember, except if they hide something like a regular series like ascending or descending hex digits.
When you say you wrote blocks of four hex characters: where do the question marks appear in the blocks where they are?