The first arms used were double bevel, like the single beveled pictured, but on both sides.  It was also marked with a drawing number B8869.  These were used very early gas trap production.  The part was revised to a single bevel but retained the B8869 number for a short period in late 1939 through early 1940.  The drawing number was left off and the part you see was born.  The next documented change was the no bevel pictured.  These were used from s/n 200,000 to the end of the War.