starter motor differences

["andy pole" <ampole@hotmail.com>]

Friends
I have 2 starter motors both lucas m418g's. Does anyone know the difference 
between a 25578 and a 25521, even the toothed pinions look the same???
The 25521 is stamped 61 and the 25578 is stamped year 66.
I guess the 25578 is correct for my bj8 but would like to use the other as 
it appears to have had a full recondition.
Many thanks in advance Andy