This rifle was probably purchased for about $15 after the war and it made sense to alter it for hunting and re-sell for about $80. It may have been proof-tested by the gunsmith and determined to be safe enough at that time. Gunsmith time and machine costs were probably about $8 per hour when that was converted, but it's plausible that he did it for re-sale, or for his own use.
Fortunately, people these days have access to relatively inexpensive new rifles, so it doesn't make much sense to repair and use the particular Jap barreled-action in question (except for bragging rights). For instance, Ruger Americans are being sold at discount sporting goods stores and Wal-Mart for less than $350.
People are like rifles. Some are tried and true, having great eyes, personality, and fun to be with. Others never seem to hit the mark with you. Still others go off half-cocked. Still, it's nice to know most of them.