I don't have a final answer, but I'll try to give my contribution to this interesting issue.
1) Seguso is a pretty common name in Murano as in Venice and there are several people involved in glassmaking with this name. Also the seguso family has very acient roots in Murano and therefore there are some different branches with their own related companies.
Anyway these are the Seguso Companies now working:
Seguso Gianni Vetreria Artistica Artigiana
Seguso Vetri D'Arte
Seguso Viro Vetreria Artistica
Seguso Viro Galleria D'Arte Del Vetro
Matteo Seguso Incisore D'Arte su Vetro
Roscano Bottega D'arte Di Gianandrea Seguso
Vetreria Artistica Archimede Seguso Srl
Seguso Gianni Vetreria Artistica
Seguso Livio Studio
I know other 3-4 glass companies with a "seguso" in the name, now closed or renamed.
2) For my experience I have seen that the main part of the signatures are applied not by one single master glassmakers, but by somebody else. Often the pieces are stocked in the warehouse unsigned and they are singed just before the packing and shipping, may be by the same people in charge to pack them. It means a) that the signatures can be not always identical in the time; b) that these people don't care too much about the "quality" of the signature (mistakes in the name, or a AV instead of a VA, et similia)
3) Anyway this signature in photo looks at least as made by somebody not very use to write with a Dremel. Considering that also if specialized of packing and boxing, every worker in Murano is used to engrave glass with a diamond drill, I suppose that this signature can be applied in a second time by somebody else, may be to add value to the piece before to sell it (so many clients ask for a signature also on small items). If I translate Art Glass in Italian looks as Arte Vetro (A.V.), but the correct translation is Vetro d'arte (V.A. or VdA).
Also: the style and the label say as end '70s/'80s not '50s.
May be a signature made in a second time by somebody not italian?