Serial no. Profile for Material Group

Hi PP Gurus,

I want to give serial no. for material grp not for individual materials.

Like we have 3 Types of mat grps DC motors , AC Motors & AC Gen.

Client wants to give serial no for entire DC motors( under this grp some 30 types of mat r there) starting with 200700001 .......... like that.

I tried for mat grps its not taking.

Thanks in Advance......

Rgds

Srinivas Rapelly