Before the Nova, there were Italian treadle machines virtually unknown in the U.S., and there were also straight-stitch electric models such as the BCJ... but the the BF and BU really started the Necchi "revolution" (such as it was) in the U.S.
These early machines came in black (most common) and a Kelly green, and were labelled, quite simply, "BF" or "BU" on the inside of the pillar.
"BF" is Italian for "Bobina Familia," and "BU" means "Bobina Universale"... the former meaning straight-stitch only, and the latter indicating that the machine could also do zigzag. Another difference between the two, as far as I know, the BF machines all take short-shank feet, and the BU machines take long-shank.
My theory is that after the Mira came out, Necchi had to distinguish between the two different body styles (the guts and functions were virtually identical) and named the earlier style machine Nova, the later one, Mira. And started putting the Nova or Mira nameplate on the front of the pillar at that point...
... and at the same time, also changed the logo on the bed from the "VN" (for Vittorio Necchi, see pix above) to the cartoonish-stitch "N" logo. I think the tension mechanism changed at this time, too. My early (i.e., BF and BU only designation) machines have the older, silver tension mechanism... and the later Novas have the same style as the Miras, with the colored button in the center.
But I'm just guessing. Would be delighted to hear from anyone with confirming or contrary information.
A curiosity that, I think, bolsters my theory; this machine:
So, you'd call it a Mira, wouldn't you?
Take a look at the nameplate:
Now if that's not weird, I don't know what is. Did this plate get tossed in, inadvertently, with a box of Mira plates on the assembly line? The serial number of this machine is also, clearly, a Mira, beginning with the letter "M."