The shape of the clasp is called "melon". I have 3 of these and they are 18K. I do not expect that it was custom made, no.
There are "pick-a-pearl" souvenir shops that open an oyster or mussel that you select, and you get whatever pearl was inside. Typically they then offer to set it for you. Perhaps your mom got a pearl or two there. I very strongly doubt this necklace was made from such pearls. Rather, I expect they were purchased at a store. They are well matched.
Silk becomes weak with age and exposure to all sorts of substances, be they lotions, pollution, skin oils etc. The silk could break and the necklace be lost. Also, with time and wear the knots become frayed and the silk stretches, leaving unsightly gaps.
Lots of men wear pearls but if that is not your style, that is okay too. My husband has Tahitian pearls he wears.
But if you do plan to wear them, do restring them.
Don't bother with restringing if you plan to sell them. The buyer can do that.
Do you have a young female relative who might like them? Maybe a niece? The pearls are petite.