Look at gunparts.com or Numrich arms
It should be marked on the barrel if it has the Marlin micro groove rifling.The model 25m was made from 1989-2003.That being said it will if it was a early made rifle.
50-150 USD
My Glenfield Model 25 has a micro-groove barrel,so I imagine others do as well.
I have seen them sell for between 400-500 dollars depending on condition and a good bore.
No
Numrich gun parts Item #418800 D Key #28 $18.45 + shipping
The Marlin 1870 is valued at a price close to $600 to $700. The exact will price will depend upon the condition of the rifle.
200.00
200.00
50-200 USD
You will need to add the serial number,to get the answer to your question.
Your Marlin model 783 was made in the year 1971.