All Marlin Ballard rifles use 1.125" (1 1/8) spacing for their tang sight. The rear barrel sights are unusual for the Marlin Ballard and the same as other Marlin pre 1890 repeaters. They were a buckhorn, but with small pointed "ears" at each lower side of the buckhorn. They used a "fish hook" or "harpoon" shaped elevator.
Front sights were standard on your gun as a German silver blade. Your Ballard in .38 Long is a #2 Hunter, so an optional tang sight would normally be the base model sliding Gallery tang sight. It looked much like the upper end Ballard vernier tang sight, but elevation was adjusted by loosening the eye cup and sliding it up or down. A true vernier could be also ordered, but not many were used on the #2 Hunter. An optional fixed globe front sight could also be purchased, as could a windage globe front. But again the more expensive windage globe would rarely be on a #2. The fixed globes are not as easy to find since not many were purchased. Most people either used the stock blade, or opted for the expensive windage globe.
These would be the upgraded top of the line windage globe and midrange vernier for a Marlin Ballard:
(You need to Login or Register to view media files and links) (You need to Login or Register to view media files and links) And the simple Gallery or Sporting tang sight a #2 would usually come with:
(You need to Login or Register to view media files and links) Another option was this flip up combination peep/post:
(You need to Login or Register to view media files and links) And the same combination, but with windage:
(You need to Login or Register to view media files and links) And a Longrange vernier:
(You need to Login or Register to view media files and links) A super rare tall Midrange vernier. It falls between the normal Midrange and a Longrange:
(You need to Login or Register to view media files and links) Can't find an image in my pictures of a fixed globe that is correct for a #2. I've got several with fixed globes, just no pictures.
The plug screws are not used to mount a base. They are simply there from the factory to plug the holes if a tang sight wasn't ordered. All Marlin Ballard rifles were tapped for tang screws whether a sight was ordered or not. The holes are 9-40, and correct screws can be purchased from MVA sights.