This plaque reads as follows but I am unsure of the 'that his' as this appears not to be very good English and so is probably wrong.
This property was presented to the National Trust by Lt. Col. G. L. Tyringham, of Trevethoe in March 1946, that his (?) wish is to be regarded as a memorial to the men and women of Cornwall who gave their lives in the service of the country during the two wars 1914-1918, 1939-1945.