The double triggers and fixed chokes hurt resale some. $400 is realistic.
No way to tell from the sn. You will have to contact Miroku
You have a Baikal Model IJ-18 that was imported from the Soviet Union. If it has a 26 inch barrel it has no choke, a 28 inch barrel has a modified choke, and the 30 inch has a full choke. It is worth about $100 in excellent shape.
It is very recreational and great activity where participants using shotgun shooting chokes. It is one of the three major disciplines of clay pigeon shooting, sporting clays and trap shooting. There are many types of skeet which include Olympic skeet or international skeet. It is designed to be a 30-inch circle at 21 yards distance.
About $75.00
At 16 yards, probably the improved modified choke in a 30 inch bbl is the most commonly used
500.00
Miroku is simply a brand name, like Browning or Winchester. The correct size SHOTSHELL (proper term, not bullet) will be marked on the barrel, such as 12 guage, 2 3/4 inch, or 20 g, 3 inch, etc). The specific shell loading you want will depend on the use of the shotgun- buckshot or slugs for larger game, birdshot for small game.
The Glenfield 50 was manufactured in 12ga from 1966 to 1979. The bolt-action magazine fed shotgun came at various times with fixed or variable chokes. The stock was birch with a walnut finish. 89,883 were produced. All had 28-inch barrels. It was produced in 20ga in 1966 and 1967 with a full choke 26-inch barrel. The Marlin 50DL was produced in a birch/walnut stock in 1996, 12ga, 28-inch barrel with fixed modified choke. In 1997 and 1998 it was produced with a black synthetic stock. I found one reference stating it was produced after 1998, but can't confirm it. The answer to your question is that while they are generally similar, they are not the same.
Without a detailed description of all markings, features, box, papers, accessories, etc.. you can be in the <100 USD to >10000 USD range.
It depends on which specific grade. Model 42's were produced from "Plain Jane" versions through special skeet versions with lots of engraving. Expect a plain one in good condition to be worth $1000, more elaborate versions could be worth $5000+.
This gun was made in 1938. The cutts can hurt a guns value regardless of who installed it. The exception is if you have the auto-5 skeet model. Check the choke marking. The skeet model could be ordered with the beavertail forend which stands out. I've seen some nice 38 & 39 guns that have been close to that 98%, but they are not getting bids over $500-700. Unless the gun is the actual skeet model, then because of the cutts, the gun is considered un-original. Unfortuantely most shooters and many collectors shy away from the front trigger safety too. Cabelas currently has a minty skeet model up for sale and they are asking $1800.