S-130 is a type of rear sight used on Mossberg rifles- but not the model of the rifle. Take a look on the barrel.
don't take down a Winchester model 52, the way they bedded the rifle was a mystery. It may never shoot the same if you take it apart.
take it to a gunsmith and ask for help
Your Mossberg model 45B is chambered in .22 S,L,or LR. caliber.As such it will take a bullet weight from 30gr to 45gr which is loaded in today,s ammo.I shoot the 40gr. Lead soft point bullets through my Browning A-bolt rifle with great accuracy.These are the Remington Thunderbolt ammo.Many a squirrel has fallen victim to this rifle and ammo combination thru the years.
I would try You-Tube for a video of the procedure first.
Your best course of action is to ask a gunsmith for help.
K Mart did not make guns- they bought them from companies like Mossberg, Marlin, Remington, who marked them K Mart's brand and a model number. If you post the model number and all other markings here, we may be able to give you the parent make and model, and a source for a magazine, And a .22 Long is not a shotgun, it is a rifle. You can also take your rifle to a gunsmith, who can cross reference the store model number to the parent model.
Contact Marlin customer service through their website, and request an owner's manual for your rifle. They are free.
Take it to a gunsmith for verification
Bluegrass was a trade name used by Belknap Hardware of Louisville, KY. The Mossberg model L was a new one for me, but I found it in the books and it fits your description to a T. Blue Book says it was manufactured from 1929 to 1932 and the value at 60% = $165 and at 100% = $450. Parts are going to be hard to find for something that had such a limited run, unless other Mossbergs would use the same buttplate. You might try Numrich Gun Parts at the related link. * Those prices are for a gun with the Mossberg name. Having the Bluegrass name will probably take a bit off the value to a Mossberg collector (if there is such a thing) but maybe add a bit if you can find someone who collects Belknap items.
22 long rifle only
You will have to take it apart. Each model is different so you will have to find a diagram of the rifle.