I will stick my neck out (and put my foot in ) to say judging by a few clues I will guess it may come from the BONATI/CONTI factory (Bonati is frequently confused with Billato, there were "brothers" in both firms but they are not the same firm) and given it's Tig-welded with a unicrown fork has to from the '90s or so...
Clues are that brazed-on "flag" on the south end of the top tube (others used it but Bonati was a "power user") and this is the "later, "Italicized" or slanted flag version, which makes the bike look faster, right?
And the semi-vertical dropouts.
But what's curveball is that paint: I'd bet money it was "Varnish by Dosenna Carlo" but he applied his one-off paintjobs to several different contractors and marques, but I believe all were in the Greater Milano Regione, FWIW. And per the website Zanella was based in Pordenone, pretty far from
Milan in the region north of Venice, so...not sure what that tells us.
Would be a great idea to run the pix (including close-ups of frame details) by Roberto Billato as well as post some here!