I could give a very, very brief thumbnail sketch of the four, but there's a detailed explanation at
www.shotgunworld.com/bbs/viewtopic.php?t=69662
It begins with...
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"F.lli Rizzini make only high quality, custom made shotguns and rifles.
B. Rizzini produces the guns imported to the US by Rizzini USA, and previously by Sigarms (Sig Rizzini and Sigarms SA5)
I. Rizzini is better known as F.A.I.R., and makes guns sold under that name and also made the O/U guns sold under the Verona name.
E. Rizzini was formerly an independent company, but the E. Rizzini line of guns is now produced by the firm of Fausti Stefano."
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And then there is Caesar Guerini. B. Rizzini is the uncle of the two Guerini brothers. They used to work for him.
(Edited to add their names: Giorgio and Antonio)
John