The stainless steel top was corroding due to salt water and various chemicals used in testing. This is a friction fit cover to prevent further damage.
Link for the mixer if you would like one of your own.
https://www.amazon.com/Magnetic-stirrer-magnetic-Stirring-Capacity/dp/B072K24X5P/ref=sr_1_4?crid=3B2UD1W41651A&keywords=intllab&qid=1693961443&sprefix=intllab%2Caps%2C106&sr=8-4
The author marked this model as their own original creation.