Its really hard to say what this gun is worth or what he model is, but I'll take a stab at it and try and justify my guess.
Specifically, in regards to the model many small Spanish makers have built guns to the specifications of the importer (customer) and side stepped their own pre-established models to meet those needs. So, you probably are not going to find a catalogue with the model of this gun in it. Today, the reason Arrizabalaga is considered a primer gun maker is because they only have one model: "Best". In regards to the gun above, I believe that it was made for the American market due to the configuration (pistol grip/beaver tail forend).
In terms of price, this was a mid to high grade gun in regards to other makers, but definitely a lower end gun for P. Arrizabalaga. That is evident by the lack of engraving, gas escape valves, or an articulated trigger. These are all indications that the gun was built to a price and is not of the same ornate level of craftsmanship of today's guns. Furthermore it is not in a very desirable configuration (pistol grip and beavertail forend). However, it is a 28 ga. and an Arrizabalaga--two characteristics that demand a higher premium. The Blue Book value on Arrizabalagas is going to be way off because it only refers to more modern models (say 1990s forward) and yours is from 1962 way before this manufacturer began focusing only on best guns. So, the price is going to be very difficult to determine. We can compare this gun to the Stoeger Victor Sarasquesta Sterlingworths from the same period which are in the same configuration (PG and BF) and similar options (the V. Sarasqueta actually has more engraving). Those guns go for about $1200 in 12 ga. Since yours is an Arrizabalaga and a 28 ga., my guess is that a knowledgeable and reputable dealer might ask anywhere from $1500 to $2000 for this gun in the condition it is in. A pristine condition gun might bring more if you could find the right buyer. Some might try and ask as much as $3000 for the gun due to the Arrizabalaga name and the gauge, but I believe that they would be hard pressed to find a buyer. Beyond $3000 there are a lot of great Spanish SxSs for sale in a more desirable configuration.
Please don't get to hung up on the value of the gun. In situations like this, price is not only tied to appraised value, but more importantly to the demand and ability to find that right buyer. A case in point, I have a very nice original copy of Don Quijote de la Mancha from 1777 that was the first DQ made using copper printing plates. I have had that book appraised for $12,000 but at one point I put it on consignment and tried to sell it for $10,000 for several years. I finally realized it was not going to sell for anywhere near the appraised price and put it back on my bookshelf. I read it every now and then and show it to my friends who are in to old books and other antiquities.
Just enjoy shooting that gun! It is a quality built gun and very nice gun at that!
