This is based on the announced design intent for the later phaser rifle, as well as taking hints from Matt Jefferies original design sketch (there's a notation for a small notch at the back for the attachment of "unit #4", presumably the rifle stock) and taking the approach that they did on the show, i.e., reuse as much as possible and keep it cheap.
So, since the premise is supposed to be something that the Phaser II would snap into and increase both power and range, it makes sense that a part of the design would incorporate additional power packs and a beefier emitter.
I've shown this to Bjo Trimble and Mike Okuda, and both said that this is probably spot-on to what Jefferies would've cooked up. I think it also helps that it's reminiscent of the Special U.N.C.L.E. gun, especially if you stick another power pack to the pistol grip, which would've been a definite plus for merchandising.