All shotgun frames are sb1s, unless its a shotgun barrel that has been fitted as an accessory to a rifle (sb2) frame. Originally the old H&R Toppers used only one frame, on which combos were available: 30-30, 22 Hornet, 357mag, 44 mag and 45-70. These old cast frames later were morfed into the sb1 frames. NONE of these Topper or sb1 frames are as strong as the current sb2 frames. And you are correct, the current H&R/NEF shop will not fit a rifle barrel to anything but a sb2. I guess the excetion is the 45 Colt Survivor, which I have been told is on a sb1 frame. And, one should remember the stength limitations when handloading for the older Toppers. In the years past (early 70s) I had both 357 and 30-30 Toppers rechambered, but was lucky enough not to get into problems. I would not do that again. Besides, I consider the old Toppers collectables now.
Wally