I think the best evidence is the design of the base of the small tiara, in both its forms. It doesn't look like the base of the Queen Victoria necklace/tiara. It looks much more like the removable base of the Girls tiara, but we know it is not that base. If it were the base of the Victoria piece, diamonds would have to have been added to make it look the way it does. Why do that for such a small tiara?