When DeTommasso (Sp?) took over a host of Italian brands in the seventies he planned to produce a whole new line of multi cylinder bikes to compete with the Japanese. Of a 'Modular' design similar to Bert Hopwood's designs for the failing British industry but with more plagiarism of actual Japanese engineering, particularly Honda, hey did come to fruition but he was overstretched.
The 254, 354,(?) and 750 Sei were all products of this time badged in several ways for different markets.
Both Guzzi and Benelli got the 250's and maybe the 350's. Only Benelli got the six cylinder motor. Strangely, in Singapore and maybe Malasia the 254 was sold badged as a MotoBi!
Pete
PS, there may also have been a 504? It was a lifetime ago.....