Standard configuration for a 'Duck' made for stock is 30 or 32" barrels, F/F choke(34 thousands) and 13&5/8" LOP for wearing with heavy clothing while waterfowl hunting. If you have never used a 'Duck' while hunting in cold weather, you will be surprised at how much the shortened LOP helps.
$6900 for a 90% 'Duck' is way overpriced, generaly thes guns come in two flavors, well used and not used hardly. Keep looking bbut be aware that many Model 21's that are like new have been refinished. Good luck.-Dick