You asked for a diagram. I hope a photo will be enough to shed the necessary amount of light about cannon-mount pintles. By the way, I should have said the Ebay item can be used as either a center-pintle or a front-pintle.
Here's an original civil war photo showing a center-mount pintle ...meaning, it is located under the center of the cannon's mount. Sometimes it was located at the front of the mount. If I recall correctly, the civil war term for that type was a "Barbette" mount.
Regards,
Pete