Why the attacked ships' origins remain uncertain

Were the Frankton targets with American-sounding names originally American vessels?

Possibly, or they may have served American-oriented lines, but the available sources conflict and do not support a firm conclusion.

American-sounding names do not settle the vessels' origins. A possible connection with American-oriented shipping lines remains uncertain. The two books consulted give conflicting accounts of the names and origins, and planned archival checks had not yet taken place.