A 1953 JC Higgins shotgun 12 gauge model 20 pump is values at about $150. It was sold by Sears Roebuck.
Yes- that is what the poly-choke is for.
j. c. higgins 12 ga auto vented rib adjustable choke
Try e-gunparts.com
Yes, as long as it has the right choke for slugs (cylinder or improved cylinder), the vents will make no difference to the slug, and will reduce recoil slightly.
Check the auction sites, local gun show, local gun shop, Shotgun News.
You don't give us the model, so it is hard to answer the question. Assuming the gun does not have an obvious adjustable choke, the choke is probably not easily adjustable. A gunsmith can open the choke (from Full to Modified for example) or can install internal interchangeable choke tubes (costs around $125).
The choke you use depends on your target rather than the type of shotgun. For example: if you are shooting skeet, you would use improved or modified cylinder, but if you are shooting trap, you would use full choke.
A barrel with a full choke.
System of changable shotgun choke tubes made by Mossberg. Accu Choke is their brand name. A shotgun choke is a means of reducing the bore size at the muzzle of a shotgun, to produce shot patterns that spread to a greater or lesser degree.
Cylinder bore- no choke at all.
You first need to identify the actual manufacturer since JC Higgins was a brand name used by Sears to market numerous manufacturer's guns including Winchester, High Standard, Marlin, and others.
Enter a search on eBay or Guns America, or Auction Arms. What you want will come up, then bid on it. Or, contact a gunsmith and have him find one for you.