Hi,
At a vague glance from what you're describing, when the mercury was discontinued, and the supersport took its place, I dare say a very good likeness to a meteor super, just a pity Bsa had nothing more radical in the design department than back tracking to and older design.
They for a very short time made the supersport delux/ custom very much like you describe , better stock raised cheakpeice, maxigrip scoperail, adjustable breech crossbolt. And increased power, ticks alot of boxs and it the same time slot early 80s,This is what it may have morfed into?