The photos are much better, but luster is still hard to tell-- they do not appear very lustrous but that could be just the problem we all have with photographing pearls. The colors of the pearls are very uniform, but are throwing me off. What color are they in person? On my monitor one looks like lavender/gray and the other looks cream/yellowish.
I wouldn't think they're imitation-- I see some surface markings, one in the pearl next to the clasp in the first photo, and in the 3rd photo from the bottom. The pearls aren't all perfectly round but most look pretty round, which you would expect if there is a bead nucleus inside. I've seen strands of natural pearls in photos and they are usually more irregularly shaped than these.
Also, isn't it true that cultured pearl strands in the 1940s/50s or so tended to be graduated strands? I am certainly not any sort of expert on pearls, just a pearl lover, but I'm going to venture a guess that these are cultured salt-water strands.
Do you have a 10X loupe (magnifier)? If you don't, I suggest you visit a jeweler with your strands and ask to use their loupe to look at your pearls. If the surface has an orange-rind appearance, they are imitation. Real nacre has a smoother appearance (except for surface flaws.) If you can't tell, ask to see a strand of cultured pearls and compare the surface appearance of the pearls.
Look at many of the the drill holes with the loupe. Can you see any evidence that there is a bead inside, under the nacre? Any places near the holes where the nacre has worn away or peeled away? On an older strand that has been worn enough to have had to be restrung, I would think some of the drill holes would show some wear.
It's slow on the forum because of the holidays, but others who know more than me will be along to tell you what they think of the strands.
As far as a wedding gift, are you thinking of your future DIL wearing these at the wedding? If so, consider that the bride might wish for white pearls to match her dress if it's white; the cream (?) ones would go with a cream colored gown though.
Does she like pearls, and old fashioned jewelry in general? If she does, and these are pearls from her future husband's grandmother, she might love to own them, but not all ladies like to wear pearls, especially older, smaller, graduated, off-white strands. I would probe a bit, or ask your son to, before making these your official wedding gift to them. if she doesn't fancy them, it could be awkward. The fashion now is for larger pearls, I think, and good quality, round cultured freshwater pearls are so affordable that many brides might prefer to have a new strand for their wedding. Just my opinion....
Also, about insurance-- if these are older cultured strands, one's ordinary homeowner's insurance policy usually covers a certain amount of jewelry. I don't think I would buy extra insurance for them.