Very Old Diamond Brooch with No Holes Behind the Diamonds

What year did they start putting holes behind diamonds?

I ask because I have recently bought a diamond brooch from an estate sale. It was mixed in with some costume jewelry and at first I thought it might be some clear saphires so i used my electronic diamond tester and it reacted like a diamond.

But what threw me off is they have no holes behind the diamonds and the diamonds are all different in size and shape and they have a cut that has no flat spot on the face of the diamond and the gold is pinned together in sections not made in a mold. There are also only a few cuts (facets). I have never seen diamonds like this as no two are alike and there are over 50 ranging from 2 ct to 1/3 ct. None have a cleaning hole.

It is beautiful and I can not find anything on old jewelry like this to reference.
THANK YOU