It's obviously a false positive though.
Not really. Not having the second 'prefix' after a slash goes back to the early days of the project.
As for the OP, my memory's a bit hazy, but I'm pretty sure this was an early example (Alabama being one of the first sets of US routes drafted) of Tim wanting to keep point labels short, and the lack of the hyphen was deemed acceptable here.
That can't be right, unless as you said, early example. The '-' has always been there, and standardizing the usual United States route prefixes was an early priority. KS7 not K-7, MI28 not M-28 or M28, etc. In any case, why lose that one char when there's an extra "US" after the '/'? Dropping prefixes after the slash goes back to the earliest "routedata.html" days.