Looks like a Pacific Bay A6 flush mount. The larger sizes and the Channel Lock series both have a bore size which allows mounting on larger butts without a stent. You can find these most anywhere, here's an example...

https://store.hookhack.com/A6-Flush-Mount-Chrome/productinfo/PBSSA6T/

Sharp looking reel seat in aluminum and I’m guessing Steffen used those due to their accommodating bore size. You can get A6 with wood inserts, but like so many woodies it’s a rather small hole and risky to enlarge that much (it’s wood).

Be aware that many fly reel seats in this caliber kinda assume fighting butts and you may have to purchase the butt cap separately if you want one. Check details when purchasing.