To be fair we are learning that a lot of movements are just swapped out during service, Tudor included.

Control and ownership are two completely different thing. OP certainly had control of the watches but control does not necessarily imply ownership. If the watches were acquired while the friend was married, they are technically jointly owned, even if the wife doesn’t know about them. And if the watches were acquired pre-marriage, ownership would mostly likely be transferred to the widow anyways. By not saying anything you are depriving the widow of making a claim to the property - property that most likely already belongs to her. There maybe some obscure rule that allows you to keep them, but making a ruling on such is the job of a probable court.

Simply put, you have both a moral and legal responsibility to turn over the watches.

There is a huge difference between the 733, which is essentially a Sellita SW200-1 and the Cal 400. SW-200s have an accuracy of 12 seconds a day, a 38 hour power reserve, and a 3 year warranty while the 400 has an accuracy of 5 seconds a day, a 120 hour power reserve, and a 10 year warranty.