You are correct, the #3 Medal (St. Lawrence Seaway) was produced in two thicknesses / weights. The thin version (mintage 3,700) weighed 12.44 grams, the thick (mintage 2,200) weighed 16.97 grams. Seems you have the "thin" version. The #18 (Pocahontas / Rolfe) was produced in a quantity of 6,000 medals (not 8,000). It is thought that a great many of these medals were melted down during the late 1970s and early 1980s when silver prices were sky high. It's hard to estimate how many survive. Scans of the original paperwork issued with these two medals is attached here as JPG files.
I just found this tread in my search of this Medal. I believe this is the 3rd or 4th from the last entree on this page. https://coins.www.collectors-society.com/wcm/CoinCustomSetView.aspx?s=8230