The picture to the left shows HM Queen Indrasakdi Sachi, Rama VI's consort, wearing a tiara that looks very similar to Queen Sirikit's modern tiara. I am assuming that they are one of the same, but on a closer inspection, it could indicate that the diamonds of the old tiara could have been reset in a more modern setting. This is of course entirely an assumption on my part, but they just look so very similar.
I have assumed that the tiara can also be worn as a necklace, and the picture at right shows what looks like the tiara worn as a necklace. Again, this is an assumption on my part. In addition, it looks as though sapphires could be interchaged with the diamonds, as seen on the upper right, but the Queen has not worn it in this form recently.