I don't think the barrel threads have been drilled out. It looks like that end plug is either threaded in using the original barrel threads, (thus the flats for unscrewing the plug) or held in using set screws placed in the three holes, (or both). The "end plug" looks like it has a chamber for a round in it, or as others mentioned, a step to hold some sort of barrel, sub-caliber device or other extention.
I don't think this is a "demill" job, but some sort of purpose built modification.
Perhaps for a suppressor?
EPENROD, will this end plug chamber a 9mm round?
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