What is the rectangular-shaped joint under the M1? What does it do, and what is its DOF?
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That diagram appears to be taken from the paper Design and development of a Mechanism of Robotic Arm for Lifting (Part 5), by M.H.Ismail et al - which is available to download as a pdf file from ResearchGate.
That paper also includes the following free-body diagram of the arm:
It is, perhaps, easier to see from this diagram that the 'rectangular-shaped' joint under M1 in your diagram:
is simply a revolute joint, constrained to rotate in a single plane, allowing the arm to be rotated on the base.
The depiction is different to the other revolute joints because it shows a side-view of the joint, rather than an end-view as we have in the other three revolute joints.