the vendors are trying to prevent you ordering parts for the wrong series, usually applying to the year '55 only as the 2 versions of 55 are common - some might call any TF year [55-59] a "2nd series" .... the terms apply only to 47 and 55, no other years
the "first series" is the model that looks the same as the previous model series, the 2nd series in any case is the model that looks like the following series .... Deco "series" = 41 to 47 1st ... AD "series" =47 2nd to 55 1st ... TF "series" =55 2nd to 59
Bill
