Silver rich has been described by me over and over here. Basically, early emulsions had a lot of "dead" grains. Grains of Silver which would not expose properly or develop properly! It therefore took a lot of silver in excess to get to a "normal" Dmax by today's standards.
This was in excess of 300 mg / ft square for film and in excess of 100 mg / ft square for paper. (Approximate OTOMH).
So, Silver rich was a name for "faulty emulsions" IMHO. Today, my own Azo type emulsion and others I read of here are using more silver than they need if they are properly sensitized.
Thick also refers to gelatin. Today, we can use ISO wash or UF wash to concentrate an emulsion. 50 years ago, emulsions were diluted by washing.
So, there are 2 factors in operation here!