I agree. I'm as guilty as anyone for calling them ALL "Marvbirds." Anything that has that 357/Firebird shape and was put out by Heritage, many of us have gotten used to calling them "Marvbirds" but I agree there is a difference.
From what I've seen, the 357's started in about 1988 (the prototype was built in 1987), and the latest one I've ever seen was a '93. Heritage stopped production of the 357 in 1991 from what I've heard. There seems to be a million stories about "what really happened" as to why they stopped. I've heard they were forced to from a Gibson lawsuit, and I've also heard they were just too costly to make, mainly due to the whole "neck through" construction, with no neck to body separation. There's also been much said about Marv legally being the only one that could work on them for a short time, as he was in on the original creation in Kalamazoo back in 1963. That story always sounded a little out there to me, as I don't believe anyone would have those kind of rights exclusively, unless they owned the design on a guitar. To my knowledge, Ted McCarty and Ray Dietrich were the only two names on the Firebird, although Marv did build them.
Then Marv started taking custom orders in later years. I've seen those run from about 2011-2015. There very well could be some 2010s out there. Has anyone seen an earlier one? Chime in as I'd love to know. To me, those later models were the "Marvbirds", and the late 80's/early 90's run were "H-357s", and there is a difference, as you mentioned. ALL great guitars though!
Hopefully more people chime in here! Interesting stuff with a million different versions of "what really happened." All great to read though!