Found this on the Marlin Forum:
"To tighten up the barrel, you have to loosen the small screw on the band at the back of the barrel where it goes against the receiver. Move the band a little at a time until it tightens up the barrel against the receiver then tighten the small screw.
Be advised that the Marlin Firearms Company as issued a Service Bulletin advising owners of the old pump action shotguns NOT to shoot them. There can be problems that require an overhaul to make them safe to shoot, and then only with light loads. I do not see the problems with the hammerless guns like yours that the hammer guns have, but the hammerless guns were included in the Service Bulletin.
To load the gun, you have to insert the shells in the slot in the bottom of the receiver, ahead of the trigger guard. The shell lifter must be in the up position to get the shells into the magazine tube.
The Model 43 was made from 1922 to 1930 in several variations. The basic Model 43, "A" grade sold for $45.00 in 1922.
I hope this helps."