FTSE 250 engineer Charter International has named US manufacturing group Colfax Corporation as the mystery rival bidder which gatecrashed Melrose's offer on 23 August.Colfax - which supplies fluid-handling products including pumps, fluid handling systems and controls, and speciality valves - has now confirmed that it is in preliminary discussions regarding a possible all-cash offer to acquire Charter."Colfax believes the acquisition of Charter, if consummated, would complement its stated strategy which, in addition to driving organic growth, includes pursuing value-creating acquisitions  within its served markets, and adding complementary growth platforms, to provide scale and revenue diversity," the US firm said.The announcement follows Thursday's statement by fellow engineer Melrose that it has upped its current offer for Charter from 840p per share to 850p per share, while also including a guarantee to pay an 8p dividend to Charter shareholders that falls due on the second of September.The increased proposal comprises 553 pence in Melrose shares and 297 pence in cash for each Charter share although shareholders will be able to alter the cash to share ratio on a mix and match basis. Charter said last week that it would allow Melrose access to its books and both firms had agreed to extend the "put up or shut up" deadline to the 21st of September to allow for due diligence. BC