The spindle/caliper bracket is threaded.
nope
That is hole that has been drilled, then had a thread 'tapped' into it so you can insert a machine screw or bolt.
no it has a 3/8 dovetail rail
Shafts are drilled deep into the ground and the gas is tapped and brought to the surface.
yes for a target scope .
Have it drilled and tapped by a good gunsmith.
The typical Marlin 39A does not have a scope. They are drilled and tapped that one can be added.
if it did not come with the provision to mount a scope on it you can take it to a gunsmith and have it drilled and tapped for one.
Check the scope mount makers web sites or call Remington.
The receiver will need to be drilled and tapped for a mount. See a gunsmith.
I seem to recall that may be the same gun as a Stevens 416. Most of those guns did not have drilled and tapped recievers. They did have drilled and tapped barrels for old style target scopes. You will probably do better to take it to a competent gunsmith and have him drill and tap it to fit a currently available scope base.
These guns were never drilled/tapped for scope mounts. Like most inexpensive .22 rifles, the top of the receiver is grooved for tip-off mounts. The mounts and rings are typically one piece, and sold virtually everywhere - even WalMart.