Hi Anita,
I often visit your site to peruse your latest PW's! I have noticed a lot of copycat "Murano Design" makers out there lately. Here is a link,
http://www.myumbria.com/Murano-Design-Hand-Blown-Millefiori-Art/A/B0013CVSSM.htm to a PW you might be interested in seeing. It looks a bit like the one you posted, but they won't show you the bottom. I hate when they do that. You should also get a profile or side view. This is listed as a Murano style paperweight, so they can get away with selling a
paperweight that is not actually produced on Murano.
I have also noticed that the real Murano weights of late are being produced with lower domes. It may be a cost-cutting technique, as they don't have to use as much glass, I do not know. But the newer PW's I have looked at coming from Murano are domed lower. I have never seen one with "NO" dome like yours. It's a possibility!
I know anybody can acquire the millefiori or murrine beads from different Murano companies and incorporate them into their own paperweights.
You have a real good mystery on your hands! Let me know, if you come up with anymore information? If I do, I will let you know.
Hope I helped a little,
Tom