It was made in 1903. Cannot see the second letter in the mark to narrow the search in spite of good images (the position of the mark). Stones are well set (the prongs). They could be rose diamonds (it’s not the colour but the cut, they are flat at the bottom).The only way to tell is electronic diamond tester, every jeweler has one.
I see the needle is missing, broken; it can be easily replaced, or even better soldered to the remaining part. But what’s left of it should be removed from the brooch by stretching needle bearing (see picture and red arrows). Applying high temperature to the brooch and pickling would destroy the enamel and the stones might pop out. Even if not meant to be soldered, removing the remaining part out of the bearing would show what it looked like to make the same needle. The needle should be at about 30 degrees in open upward position against ``the ridge`` of the bearing to create a spring effect, otherwise it would be loose.