Not sure why the four year overlap. Maybe that long to finally exhaust all the large BH flattop frames... Also could still have had a small market for the flattop during that time... Probably never know for sure. Looked through books that I have, and couldn't find 'overlap' reason. I know I prefer (for example) my 50th Anniversary .44 Magnum flattop over my SBH... So it goes.
Thing to know about the '.44 Magnum' is the OM and NM are both on the large frame. Basically there is no difference between a large frame BH and SBH. Grip frames, hammers, triggers, Ejector housings are interchangeable. I know guys that prefer the dragoon style grip and will place them on their BHs for example. The name 'Super Blackhawk' is really just a 'marketing' thing. I own a SUPER Blackhawk.... Rather than (rubbing toe in dirt and mumbling) 'I own a Blackhawk'.... Sounds better doesn't it?
With the .357 it was a different story. The OM BH in .357 was on the 'medium frame' (flattop and later the one with ears around rear sight). .357 was on the medium frame until the introduction of the NM when it went onto the large frame joining all the other BHs and SBHs. Then, not that long ago, we got the .357 back on the 'right' medium frame with the 50th Anniversary flattop, and of course the Lipseys flattops followed.... So it has come full circle. Note that the .357 BH large frame is still in production as far as I know.