Just to clarify Bill.... what we Brits call STASS is the same item the US types call HEED III.
The basic item is marketed by Submersible Systems in Ca as 'Spare Air', a redundant backup for divers. Although the only real differences between the Spare Air and STASS/HEED units are the Spare Air has a flexible top plate/button where the STASS?HEED has a rigid top - to prevent inadvertant partial purging, and the dial gauge is just a basic 'pop pin' device on the Spare Air (it can be replaced with a dial gauge).
The units are generally found in the normal 1.7 cu ft size, however some nations do use the larger 3.0 cu ft version. The only difference being the longer tank (the STASS pocket on an LCJ would be roughly 4in longer). As yet I've only seen the 3.0 cu ft version used by some Dutch crews.
Out of interest, I've heard many divers slating the Spare Air unit as almost useless... they're thinking more of being out of air at depth and requiring stops on the way up. From what little I know, they're probably right. But from an aircrew point of view, where it's more a matter of hit... settle... egress, it does seem to be a damn good idea.