In all my readings, the only IBM carbines that did not have BE-B or B-EB stamped trigger housings were some in the first few thousand during early productions. Some of those early IBM carbines had trigger housings from National Postal Meter which had an "N" on the left side and a "9" on the left side. For your 384xxxx IBM, I would expect a stamped / brazed trigger housing that was marked BE-B or B-EB.