The WS code indicates the engine is a 389 cid with 360 HP and a Manual transmission.
On the distributor pad is the production number which should be 9778789 also indicating the year of 1965 and 1966
The WS letter code was used until 1970.
The other codes normally seen on the front of the engine were the engine production code and the engine vin code. They were not added until 1967.
The 65 GTO shared a VIN with the lemans, you can verify the GTO option on the Data Plate, check the accessory code for a "5N".