Welcome to the forums
I take it your new online, firstly DONT give out your serial numbers for your rifles without putting atleast TWO "Astrics" or " * " instead of the last few numbers.
Secondly, this is the website of browning with that model shotgun,
You'd need to look it up based on the specific model of the shotgun. If they are removable chokes it should list on the choke what it is (Dont know what the Citori uses sorry).
If you have calipers, a telescoping gage and OD micromter, or a bore micrometer that is small enough you can always measure the bore to find out what size.
The 20 Gage is .615" so a Cylinder choke should measure .615", a Improved Cylinder should measure .605", a Modified should measure .595", a Improved Modified should measure .585" and a Full choke should measure .575". These are aproximations, there is toloarnce on these sizes as well as companies making the chokes alittle different which might result in a different sized choke but these are the "standards".