Jeff,
Here's the VUCC rule on the grid square:
7(b) For the convenience of the Awards Manager in checking cards, applicants may indicate in pencil (pencil ONLY) the grid locator on the address side of the cards that DO NOT clearly indicate the grid locator. The applicant affirms that he/she has accurately determined the proper location from the address information given on the card by signing the affirmation statement on the application.
For more proof, you can simply use qrz.com. In the case of W6RO, it does not show the actual location of the ship but you can do so by using the google map for grids and find the ship on the satellite view. This will spot the lat/long and translate into a grid square. You only have to certify that you have determined the correct grid square; you don't have to prove it to the card checker.
It's always good to go right to the rules when there is any question. Don't assume that the card checker always knows them. You will find that you will get other cards over time that don't have the grid square on them. Simply determine what the grid square is and follow the rule above. It's always a good idea to print out the VUCC rules and take a copy with you when you get your cards checked.
73, John K8YSE