I found this in some silver bullion rounds I bought recently..and thought it was pretty cool. It has a proof finish on both sides,is 1 troy oz. of .999 pure silver and on the reverse has a model 640 depicted. I did a search on a few sites and could not find another example..so maybe not "rare" but not all that common? I wonder if they were made for any other models as well. I'll have to keep my eyes open for them. 
