I think Theo is right. The tonnage difference was caused by the conversion to a SSV and the name Edgehill was perhaps used by the Royal Navy, like the name ATIK was used by the US Navy for the American decoy ship CAROLYN.
This means that the ship had still the name Willamette Valley painted on her side, but was referred to as HMS Edgehill (X 39) by the Royal Navy.

Thank you guys for the help

Regards
Rainer