I would suggest trying a local gunsmith who has been around a while. Gun shows would also be a possible source. Numrich Gun parts, http://www.e-gunparts.com/, found a clip for a Savage Model 1903 .22 short pump action rifle for me many years after it had been discontinued. They are known for their large stock of gun parts. Magazines for this rifle are also available at www.midwayusa.com. In googling the model 42, I learned it was a rebranded Marlin 80 and I bought the Marlin 80 magazine; it is product # 635389 and it looks, fits and operates the same as the original JC Higgins clip. The above is correct. The Model 42 and the Marlin Model 80 use the same clip. Do a google search for "Marlin 704046" and you will see a bunch of online stores selling them for about Fifteen bucks. midwayusa.com didn't have them when I searched.

depends on which model it is, how old it is, new would be 300.00 to 375.00, used in fair condition would be 200.00 t0 300.00

Depending on condition, $120-$160

Usual problem is worn/broken parts, or dirty gun. If the sear/trigger is worn, it can result in doubling. See a gunsmith.

I'd start with Amazon. Your rifle is a Savage model 340, and if you search for Savage 340 scope mount, you will get a lot of hits. The mount and a set of rings is about $53.98. Shoot safe and often.

There should be a manufacturer's serial number somewhere. Unless the serial number has been deliberately erased!

The W.H. Davenport Firearms Company of Norwich CT were makers of single shot shotguns from 1881 to 1900. Acme was one of Davenport's brand names. These were not expensive guns when new, and depending on condition would be worth $60-$90 now.