Dying relative wants to buy me a Rolex to remember him

[Fiery]

Sad story, but also somehow beautiful. He must be an amazing man. I know you said no flashy, no gold, and I'm also not sure how much he can spend on such a purchase. Obviously it would be a bit selfish to go for a gold model, but as others suggested, the DD would be a perfect choice. If you get it in white gold, with e.g. a silver sticks dial, it's not flashy at all. You have quite a few steel Rolexes already, so the DD would always be a special piece in your collection, especially if you keep your base collection steel in the future as well.

At the other end of the scale, for a WWII veteran a fitting model would be the AK. It's the least flashy, possibly one of the least common Rolex models. If he had a Rolex, it would probably be an AK.

I'd pick 2 or 3 models that would be good candidates, and let him choose at the AD the model he wants to leave for you.