"The great thing about standards is that there are so many to choose from!"
Your diagnosis may not be correct.
Figure 1. Standard 3.5 mm jack pinouts. Source: Prohardver.
... for a four-pin version you might use the rectangle as a fourth pin.
The four-pin plugs are known as TRRS (tip, ring, ring, sleeve) and the pins named as T, R1, R2 and S.
Update after datasheet link added.
The datasheet states that it's a 3-pole stereo jack socket - for the first plug in Figure 1.
Figure 2. Photo of the socket from the product page. Note that the sleeve contact forms a strap across the socket holding it firmly to the PCB. There are two terminals marked '2'. See Figure 3.
Figure 2. PCB pinout from datasheet.
The datasheet shows that two is connected twice and that it's closest to the connector entry point. Again, your interpretation is correct.
Figure 4. The rectangle is probably a hangover from the days of the open-jack. These are commonly found on guitars and amplifiers. The pin contact diagram is incorrect in that case as it shows pin 2 furthest from the jack entry point.
The circled region in Figure 4 was the contact for the sleeve. It appears that the modern diagrams are leaving the rectangle there but not showing a connection to it as the barrel is plastic.
The diagrams are not clear. I think you need to test it when you receive the part!