I bought two modules from the following: https://www.alibaba.com/product-detail/ThinkPad-TrackPoint-Red-Cap-Mouse-Pointer_62315777485.html
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(https://i.imgur.com/wTdlbCz.jpg)
The module is not like a picture in the page but the module in the video on the page. The smaller module is soldered on the board with Elan EM78F611 USB MCU.
It works like a standard mouse.
One of two is broken and when I push the bar to the left, the pointer moves up and then move left a bit. When I push the bar, strange noise is came from the neck of the bar. It looks the base of the bar is not well fixed on the PCB.
Another module is works well.
Diameter of the center bar is 2.5 mm, height is 6 mm from the top of the PCB.
The module does not have any screw holes on the board so its hard to fix in the keyboard. You need another USB connector.