Can anyone tell me why or when beretta shipped their 1201fp's with 18 inch & 20 inch barrels? I've seen both.

I believe they are shipped with 20's but I had an 18" that was a law enforcement turn-in that was cut down to fit the cruiser mounts.

Bought a 1201FP new about 17 years ago, most painful shotgun I ever owned. After five rounds of OO buck or slug my cheekbone would be throbbing. Sold it to a buddy who brought it to a get together shoot with friends last year, shot it again with slugs, handed it back and said that's why I sold it the first time!  Beautiful  fit and finish, never jammed, just plain hurt to shoot. Bought a Remmy 11/87 and still have that one. Bought a VersaMax tactical last year, REALLY soft shooter.

I have two 18" 1201 barrel's, the one shotgun is a later 1201fp with ghost ring sights,  Mesa M1/M2 stock w/ limbsaver pad on it, Dave's oversized safety,gg&g charging handle.  The other is my mutt it has a 18" 1201 barrel with rifle sights, M1 super 90 reciever, 1201 7round tube,  M1 carrier, M1 preban trigger group(ghost load)with Dave's oversized safety,gg&g charging handle, Mesa stock, Mesa 6rnd side saddle. I bought the one as police trade in  parts gun and was not complete(rifle sights) the other was factory Beretta gun with ghost ring sighs, and the M1 was police trade in with no magazine tube or barrel. I got all three for less than $900 and $300 in upgrade parts. The best upgrade I made is the Mesa stock, with the limbsaver because they do kick like a damn mule, highly recommend this upgrade.

My 1201FP is Detroit police surplus and has the 20" barrel. It can be ghost loaded.
It came with the standard too-long Buttstock with hard rubber butt plate, but I shoot it with the Choate PG stock. I keep it short, no spacers installed. Yes, it has some kick, but so did the Remington 11's I shot for years, and so does my Mossberg 500.
The 1201 is a light, reliable auto with good ammo capacity.