I think you'll find the Marlins are 8-40 thread.
The 'Late Model' 39A scope base is the double rail type. That has 2 sets of rails on the same base. One for the smaller dia scopes using the tip-off style mounts like most .22 scopes. On the same mount, they just milled another set of rails to take Weaver style rings so 1"d scopes can also be used with the same mounting base.
The 'Early Style' is the one like you have ,,same idea just a bit narrower as it is machined to take just the .22 tip-off style scope mounts. We used to call these the cantelever mount.
I can't remember when they went to the later style,,probably mid to late 70's is my guess.
The screws for the early style I believe are 8-40 thread,,1/4" thread length.
Weaver Style oval head mounting screws. (Two types,,high and low oval head. Low for the 39 and the very slightly higher dome style for the 336, 444, ect. But the latter will work just fine in the 39 mount. Most wouldn't even notice the difference.
I think I have one of the bases and a few of the screws yet from my time there in the early 70's. They threw anything out that fell on the assembly room floor. It was a gold mine for parts.
Check your rifle to make sure of the thread size.. and if it's 8-40,,I'll send you some of the screws.
FWIW,,,MArlin usually shipped the scope and mount (or a sling if included in a package deal) unattached to the rifle. That avoided them having to pay Fed Excise Tax on the final value of the rifle+scope+mount.
Instead they paid FET on the rifle only
