From Gun Parts Corp. website...
Jordon, A Sears Model 103.2840 is actually a Marlin Model 80. The Sears/JC Higgins Model 42 DL is also a marlin Model 80 and our Host, GPC in it Illustrated Parts Breakdown and parts list for the Marlin Model 80 on pages 439-440 list the magazine as Item #403080 a Marlin 80 Factory 7 Round .22 magazine that fits these Marlin Models:20,25,80,780 and Ranger 36. You can pull up the Illustrated parts breakdown and parts list for this model, on-line and make a visual comparison between your rifle and the Marlin...or...you can wait until the magazine you ordered comes in and try it..if it isn't the right one..you can always return it!
Magazine shows in stock.
https://www.gunpartscorp.com/products/403080B