What is the difference between a tang/pin buckle and the deployant/folding clasp buckle?
The Tang buckle is similar to a traditional men's belt buckle. It will often leave a crease in the strap, but it is closer to the original style of buckle used by the members of the Italian Navy.
The deployant, or folding clasp, will not leave as much of a mark in the strap and it less original. It also has a slight tendency to make the strap fit a bit loose, and so if you are struggling to keep your Panerai on in the first place either skip the deployant or hit the gym.
If you use the OEM (Panerai) deployant, you'll need to buy a strap that's specifically designed for use with the deployant (regular straps as well as aftermarket straps are too thick to fit through the slot in the deployant buckle). The OEM deployant straps are thinner at the buckle end to accommodate the deployant.
So make sure you read the product description when purchasing a strap. If it says for a Tang Buckle or sewn in buckle it cannot be used with a deployant. One more thing to clear up. A Deployant strap can be used on a tang buckle, but a tang strap cannot be used on a Deployant buckle.