OBS & OBF are Beretta "catalogue codes" for which set of chokes comes packed with which gun. Different models ( i.e., Sporting & Field) come with different choke sets. I've seen a number of Beretta catalogues with a choke code table, however I can't put my hand on one at the moment. (Found & the following edited)
The "OB" is Optima Bore (over-bore, approx. .733" diameter) and MC - Mobil Choke (standard bore, approx. 726") as was referenced earlier. As I recall, the Optima Bore and chokes were introduced in 2000, initially in the DT10, a year or two later with the 682GoldE and other select models including some of the A-391's (Optima Bore models) following. Many Beretta models still utilize the time proven Mobil Choke system (like the 686E), while the Optima Bore system has also been made available with some of the Custom Shop guns like the SO-5.
From the code list I have, for the 391's, the "S" signifies "Sport" and the "F" Field. The OBS choke sets include Skeet, Improved Cyl, Modified, Imp. Modified & Full. The OBF Field sets include Cylinder, Improved Cyl., Modified, Imp. Modified and Full. The OBSP "Sporting" sets are the extended, colour-coded variety, and are supplied in Cylinder(blue), Skeet(red), Imp. Cylinder(yellow), Modified(green) and Imp. Modified(black). Additionally, other OE Optima extended chokes are available; US Skeet(purple) Light Mod.(orange) Light Full(grey) and Full(white).