They are different in shape, and mechanism design, and the Armstrong one comes 8-10 years after Williams S-50. so, just coincidental, perhaps because 1/2 is .50 inches, or just because sales folks liked the number 5....
Ironicly, both were followed by short lived model 51's...(perhaps mechanics don't like the number 1)
(It is strange how much more refined the Williams one looks than the Armstrong one, given it's greater age...)