Honey Pot Bots
Meet @ShawneeDeaver. This account's first tweet - and only non-reply tweet - is 2scEY0T, an apparently random 7-character alphanumeric code. The rest of the tweets are replies sent within hours of its creation; the collage is representative. (Thanks @OlgaNYC1211 for the lead). 
We decided to look for more accounts like it. We started by harvesting the recent replies to the accounts that @ShawneeDeaver replied to, and filtering the results to accounts with a 7-character code as their first tweet and all subsequent tweets being quickly-launched replies.
We then repeated the process on the resulting accounts until hitting the point of diminishing returns. This yielded 176 accounts matching the pattern - both the naming scheme and the profile pictures show definite patterns.
Here is the combined tweet schedule of the 176 accounts. Although all tweets are sent via the Twitter website, due to the repetitive nature of the content and profiles, it's not unlikely that both account creation and tweet posting is automated.
These accounts post repetitive content, but make some effort to disguise that fact from spam filters by varying the attached image, the set of emojis used, and characters used in the text. The latter aspect is shown here - note the use of different versions of "e", for example.
Here's the language breakdown of the accounts targeted with replies by this network. Most are English, with Spanish coming in second. Most (but not all) have 50K or more followers; many are verified accounts.
One last note: we did observe possible indicators of Russian association (witting or unwitting). Providing additional detail at this time would likely prove non-productive in the long term.
Thanks @ZellaQuixote for aid with strategy and direction.
