to the forum HZsm, I think it maybe what they call an ambulance pack, with a bit of luck HK1837, or someone who is more knowledgeable than me, will see this question, and be able to answer it, if it was M it would deffo be a sandman with the 18V trim. It still sounds good even if it isn't a sandman, can you post up some pics of it...
Cheers J