The Power Cell side of the Blastek E-11 Blaster. As this weapon was actually a form of a British Sterling, the magazine clip (center of gun) was called out as a Power Cell. Makes sense.
The detail of the Hengstler Counter is incredible - seen behind the magazine receiver. This counter was used to give the gun a little more personality. Actually, as I researched the various parts used to customize a Sterling for the movie, I've been very satisfied with the level of detail and accuracy given to all parts. Speaking of personality however, it's the folding stock assembly that really makes a detailed version of a Stormtrooper Blaster stand out. In this model, the stock is not extendible.