If your Grandad's shotgun is a 183K (no other letters after the K) it was made from 1953-1956. At that time, it was assumed that you did not need a big book to tell you not to put the muzzle in your belly button- and instructions were short and sweet. Here is a link to what instructions that there were that came with the shotgun when new: http://www.box54.com/inst%20183K.jpg
The field strip- check that the gun and magazine are empty. Open bolt, pull to rear. Press and hold trigger, slide bolt out of rear. Use a coin or wide blade screw driver to loosen the take down bolt under the stock. Remove action from stock. STOP. That's far enuff, pardner! Use a toothbrush and solvent to clean what you can see, a rod and brush for the barrel. For outside, soft cloth, light oil, rub.