Abaddon,
Your first statement was correct. ---- Both rifles have pistol grips and accept detachable magazines. (The rifle in the top picture happens to have a magazine fitted, while the magazine well is empty in the second example.) Therefore the addition of any one of the other forbidden items would make either of them "assualt weapons."
With that in mind, the small doo-dad under the front sight on the rifle in the top picture actually is the preban M16/AR15 bayonet mount. Also as you probably noticed, with the 24" barrel, (as opposed to a 16) it's rather useless for actually attaching a bayonet, and has evolved more into an attachment point for flashlights and other equipment.
*****
XQ: So sorry, I was actually trying to clarify to "non-gun people." For "gun people" stating that they had the same lower receiver was more than enough. (The lower receiver actually is the firearm under statute --- Everything else is an "accessory.")
Detachable magazines, which are manufactured and (usually) purchased separately are irrelavant in and of themselves in determining whether the firearm in question is an "assualt weapon." At issue is the ability of the firearm to accept a detachable magazine.
The magazine you see in the first example is detachable, and would attach just as happily to the rifle in the second example . Except for the bayonet lug, the two are exactly the same rifle.
*****
If making an issue out of an archaic throwback to the days of the pikeman in the middle ages seems capricious, arbitrary or irrelevant, I agree. Although the AR15 is ugly and looks very similar to the military M16, (which truly is an assualt rifle) it is no different in the performance of its true function (firing bullets) than a number of traditional sporting rifles with familiar wood stocks.
Tom