Welcome to Shotgunworld, Just Bill.
Well darn, your very first post and you have already stumped me! It must not be hard to do.
I'm not familiar with the Beretta SxS guns. I also noticed that statement "Optima-choke tubes on Mobilchoke barrel" and wondered how that was possible.
I just don't know the answer, but not knowing rarely stops me from expressing an opinion, so here goes:
My guess - and it is only a
guess - is that this model has true backbored barrels. In other words, Beretta may have taken barrels designed for the MobilChoke system and bored them out to the Optima bore diameter, so they can fit Optima Chokes. The result would be thinner barrel walls and lighter barrels. This is in contrast to the Optima barrels on semi-autos, which are larger in their OD as well as ID, and therefore are heavier than the MobilChoke barrels.
It is easy to determine which kind of tubes the gun has. Just compare them to this pic from the Beretta USA website:
I assume your choke will be marked "Optima-choke". You said they pattern right, so I assume your bore is about .733, not .723. Try to find someone who has a bore gauge and can measure the bores for you.
But you know what they say about "assume": it makes an "***" out of "u" and "me". The real answer requires that you call or e-mail Beretta USA and ask them what the heck that means. You can e-mail them by going here
http://www.berettausa.com/customer/support.html and hit the "ask a question" tab.
If you get an answer from them, please let me know. Thanks.