To find BOM for a item corresponding to Purchase order

hi,

i would like to know the BOM details for a purchase item in a Purchase order. I do not want the current BOM details. i want the bom details when the pur ord was created.

eg.

Pur ord no. 111

date 01.04.08

item = M (material)

I wanted the BOM for M on date 01.04.08 ?

regards