IBM Barrel Flat Marking

Looking for information on IBM barrel.

Has anyone ever come across the marking shown in attached picture, nearest the gas piston on IBM barrels? Note; this is an 8 -43 dated barrel with no FB & san-serif P proof.
Have dug through CCNL's, books, google searches etc. but little luck.
There is a similar barrel described in CCNL #110 found on a 3.66 that was inspected. Any more information and or pics of that specific carbine referenced would be fantastic.

Thank you and Happy New Years