I was a collector of vintage Italian pens for a long time, and it seems that every single ebay ad for an Italian pen suggested that it was either a subbrand of Omas or Montegrappa. I remember one fairly prominent advertiser claiming that Tabo was a subbrand of Omas!
It may have been that Montegrappa sold some unbranded pen parts, but we don't have particularly accurate records. What we do know is that there were hundreds of pen manufacturers back in the 1930's and 40's making pens that looked very much like one another. In particular, the pen industry of Settimo Torinese was loaded with garage-type pen assemblers, sharing clips, caps, cap bands, nibs etc with one another. Many of these no-name pens looked very much like (actually, blatant ripoffs of) Montegrappa and Omas pens. Ring tops weren't all that common - often called "princess" pens.
For the most part, Montegrappa was a very low-end manufacturer, much of what it made was absolute junk. The "Extra" line of theirs was sometimes an exception, but having a vintage Montegrappa pen is no pedigree that you would want.