The d426 is a parallel action snare drum. I don't think the buttside would have looked like it does on your drum, nor do I think the bottom rim would have looked like on your drum. I am quite sure it was a seamless design, too.Here is a better pic in a 1967 catalog./Magnus
in the '69 catalog it indeed states Seamless metal shell. So the shell may not be the same type. The bottom hoop you're right, on the parallel action snare would not have the 2 holes for snare string, because the snares all go across the surface much like a Ludwig Supersensitive model, so they would have to design the hoop differently I suppose. I would like to see a 1970 or 1971 catalog scan to see if this model would be in there. It seems that the next catalog scan is around '75 which is later lugs design.
A cursory search on Yahoo images for "sonor chrome snare" and this, same drum (except this pictured drum apparently has original Sonor snare wire) was found here: http://www.drumatix.com/lf-sn-159.html
Also in the description they called the throwoff a "beer tap" design. Additionally the hoops are described as "die cast" even though they appear the same as mine, they look die cast upon initial inspection but they're triple-flanged hoops.