Try e-gunparts.com
At a good gunsmith's shop. Maybe Numrich's Gun Parts (search gunpartscorp) For the complete trigger guard assembly, including trigger, safety, springs, pins etc., you can get it from Numrich, Midway USA, Brownells or direct from Marlin. If it's just the trigger or other individual trigger guard parts you need, you'll have to get those from Numrich for a different model Marlin.
gunpartscorp website.
The bolt is removed from a Marlin model 81 by first opening the bolt completely, then while holding the trigger back all the way, continue to pull the bolt the rest of the way back. Sometimes it is necessary to hold the trigger back while re-inserting the bolt.
You need to find a copy of Brophy's book on Marlin History
marlin mod 81 dl 22
No, a Marlin 881 stock cannot fit in a model 81. The stock would fit an 881, not an 81.
Marlin 81
The trigger guard is available from Numrich Gun Parts. There are two different types of trigger guards for the Savage 6d. The plastic guard that has a inletted area for the trigger guard in the stock and the simple bent metal trigger guard that just screws into the stock wood. Plastic trigger guard is product # 103530U $24.10 Metal guard is product # 10543OAL $11.50
Your Model 81 was halted in the year 2003.I would surmise that Marlin had a new model in the works at the time of the model 81 demise.
The serial number is located about 3 1/2 inches forward of the center of the trigger guard on the left side of the rifle.
try looking under the trigger guard. That's where I found my Model 99 M1 Cal-22LRserial number
If your Mdl 42 is a bolt action 22 rifle, it is a Marlin Mdl 80. There were 5 different trigger guards used at different times. Go to the wbsite for gunpartscorp, select M, then Marlin-Glenfield. Model 80, scroll down to part 78. Some of them are in stock.