My first guess would be Nupla; however, most Nupla soft face hammers I have seen have grooves around the aluminum head like the Williams-branded version shown below. I've also seen Nupla-made soft face hammers branded Proto, SK, and Bonney. (note this pic is from the future)
I suspect that the ones without the grooves may be earlier. I checked another early catalog (1951) and it appears that Williams was buying them from Nupla. So everyone is correct in their ID of the manufacturer.
-Don