This is the old version of the Joint Lock /Pole Clamp module for MDM10- and MMS10- Series Desk Mount Monitor Arm Bracket. They connect the arm to the pole. The difference with the new version is they lack the quick-release function (for height adjustment).
They fit a 35 mm diameter pole. If you want to add more monitors to your existing MDM11S or MDM12D/Q desk mount monitor bracket, then you must have these joint lock modules. For one monitor, you will need a single joint lock, for two monitors (side-by-side in one row), you will need a dual joint lock.
Notes: these joint lock modules will not work for the MDM0- series (MDM03, MDM04, MDM05, MDM06, MDM07) desk mount monitor arms because the pole diameter is different.