Here's some good sources for Marlin 39 parts.
Here's a good exploded view to help you positively ID the parts and names:
http://www.stevespages.com/ipb-marlin-39a.html
The 1946 version Marlin 39-A didn't have a cutoff spacer. That was a later addition. Your version has a spring cut off with a thicker end. This type of cut off is simply attached in it's slot in the receiver with the screw. If you can't find an original type of cut off, you may have to use a newer part and a spacer.
On these early post-war rifles virtually every part, including the flat springs are milled steel.
At the rear of the receiver, under the stock is a mainspring tension plate. This positions the mainspring and hammer plunger, and is adjustable for tension. By moving the tension plate forward or back in the grooves cut in the tangs, you can adjust the hammer tension.
The earlier front sight hood is not as wide as the later hoods.
Here's parts:
http://www.gunpartscorp.com/catalog/Models.aspx?catid=310
http://www.jackfirstgun.com/
(Gotta call but they're GOOD)
http://www.brownells.com/.aspx/pid=0/sid=283/schematicsdetail/39A
(New parts for the current rifles. Some will fit older rifles).
http://www.gun-parts.com/index.html/
http://www.poppertsgunparts.com/index.htm
http://www.poppertsgunparts.com/index.htm
(Click on Marlin and WAIT).