A lot of money is lost via trappers not properly stretching their furs. Stretching beaver round lose both size and fur density. The last place a beaver primes is in the back of the head. You will notice as the pelt becomes prime the leather becomes white at the edges first and slowly changes colour until the whole leather side is white. The center of the back on a fall beaver is going to be much darker in the leather. This tells you that the underfur is not complete yet. When a beaver is graded we check the leather first then the fur on the flanks and the last place we check is the back of the neck area. This area will tell you the underfur is complete or not. Stretching a beaver round pulls the fur away from that area.
I will clearly state for the record the proper stretch for beaver is the oval stretch. You are free to stretch anyway you want but for the record the oval stretch is the best stretch for beaver. With the nose and whiskers off.