Pitting is the corrosion holes, starting shallow, from rust in the bores. You need to go over the bores with a bronze brush to ensure that any leading is remove to ensure that hidden pits are not filled with lead.
Your estimate of choke is right on - 0.5 mm constriction of choke is approximately 0.020 thousandths of an inch constriction and 0.7 mm is approximately 0.028 thousandths of an inch choke. A quick measurement at the muzzles with a micrometer caliper will confirm whether the chokes have been altered since proof. Date Code N*1 indicates 1968. An e-mail to AyA would confirm their model designation in the records.
I have a Model 106 AyA 16 gauge that is very similar to your gun, but with extractors, which I did buy from Tradeex for about $350 net including shipping. The barrel length is the same on mine, but the chokes are Extra full and Improved Modified. Mine has a factory original buttstock. I like my AyA 16 gauge and seem to be able to shoot it well.
An ejector gun is definitely worth more than an extractor gun, but the shortened buttstock does hurt value. Many shooters would use a shorter length of pull than 15 1/2 inches so they might remove some of the spacer, thus making the buttstock more attractive.
Ultimately, the market will tell you what the gun is worth. You can always price the gun a little higher than you think it might bring, and "test the waters". You can search the Internet for comparable in Canada and the US, recognizing prices for side x side shotguns are higher in the US than Canada. Modest double guns like this are common and inexpensive at this time in Europe, if you search guns located in Norway, Sweden and Germany.
You have a serviceable Upland bird gun, assuming that everything mechanical works as it should, but comparing it to others that I see in Gun Shows, I think that you might get $400 or slightly over that on a good day. The market is not strong for modest fixed choke, not steel shot proofed, side by side shotguns.