I am just curious to know what did you rely on, on what have you based your thoughts to place the ( "5AB7" ) on the string ?
the second hint was: The first "?" = NO.50 = 5AB7
If you take a look to the image there are 4 variables on the blue line, and two '?', if the first one is NO.50 = 5AB7 and the chars we are looking for are 4, then 5AB7 should be the chars and they match because we are looking for Hex chars.
Why this position not another?
Because the first '?' is the 3rd position in the blue line.
And what make you think that is for substituting of the (XXX..) characters, not something else ?!
I'm just following the colors pattern from the Image.
A good question is:
What's the relation of NO.50 with 5AB7. If someone can answer that then the key could be find with brute force.