If he wanted you to keep them, you'd be in his will, or he would have clearly indicated that it was a gift to you.
If he doesn't have a will, it's not yours.
Give the collection to the rightful owner, tactfully. I'm sure she'll wonder why you had it, and it wasn't at the house. Have a respectful answer.
If you want them, offer to buy them at an appropriate time.
If he wanted you to keep them, you'd be in his will, or he would have clearly indicated that it was a gift to you.
If he doesn't have a will, it's not yours.
Give the collection to the rightful owner, tactfully. I'm sure she'll wonder why you had it, and it wasn't at the house. Have a respectful answer.
If you want them, offer to buy them at an appropriate time.