It only takes "ONE" bolt to hold a double shut. Most doubles of moderate price & downward when fitted with multiple bolts have one which actually does all the work. The top bolt is further from the hinge pin than an underbolt thus has a mechanical advantage.
A couple of guns of renown using a single underbolt are the Parker Bros & WW Model 21.
Some guns of note using only a top bolt are Lefever Arms Co, D M Lefever, Baker, Fox, L C Smith, Syracuse Arms, Ithaca NID & Nitro Special (branded Lefever).
Nothing at all wrong with the concept of a single top bolt. Any shortcomings of the 311 are likely in the mat'l or fitting, not the fact it is a single top bolt.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra