OK lets make a stab at these, very difficult without actually seeing them, etc. Max, it could be Seguso. but most of his vases that I've seen in this style have more colour with the gold as the secondary colouring. Not ruling Seguso out, but could also be b y Barovier and Toso from the 1950's and they did some similar work.
Pip I just don't get a Seguso feel to the shell and think it's probably Barbini or Fratelli Toso, both of whom did this type of work with shell designs.