My factory Beretta Optima flush choke does not look like what you have pictured.I believe it is a field gun. The serial numbers on the receiver and barrel do NOT match.
I have already figured that I do not need Optima HP, because I tried the chokes from my 686. My decision is between Optima and Optima Plus. When looking at the threading of a flush Optima choke, there's no way it would work with the barrel threading. The optima choke threads start at the very top of the choke...where the Optima PLUS chokes have that same gap before the threads begin (just like my pictured barrel).
The threads for an Optima choke do not go quite to the end of the barrelCertain 391 field models came with optima plus chokes. I have owned several.
If the barrel has a narrow rib and doesn’t say sporting it could be optima plus. There was a time when 391s used Mobil. Optima plus, and optima. The optima (with the threads at the end of the barrel) were in sporting (wide rib) models.
Not one of Beretta’s best marketing moves to use the same name for 3 different choke systems.