If you don't know which letters are on the front of "TN6789", or even how many letters are on the front of that string, you can change the substitution to this:

 s/^\D*//
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...where ^ matches the start of the string, and \D matches "not a digit", and the * applies to "not a digit" and means "0 or more times". Because the * is greedy, it will match as many non-digits as it can find at the start of the string.