It's the baton the manhacks used.
Whoa, that's just a whole mess of wrong.
First of all, those are Combine soldiers (Well, Civil Protection Officers), not manhacks (Which were the flying buzzsaws). Second of all, that baton is called a Stunstick (Which will also be added to HL2 DM). Thirdly, the "Slam" was, in the leak at least, a satchel charge.