Sounds like you found one of those very scarce to find Dummy Mags ! If fitting to a Jackal action that never had one fitted - only a few variants ever had them fitted ... as prev poster suggests , line up and mark before drilling .The original used self tapping screws but if really nimble some no.8 size nut and bolts could be used .
If fitting a bipod , make use of the hollow stock 'slots' that you note - no need to drill anything , and use a hollow stock bolt as supplied with some Swivel Sets as standard ( a larger washer will help spread the load better ).
Use the long slot and before tightening all up slide the bolt back and forth a bit with whatever bipod base fitted on to get a perfect fit . I had a Ramsbottoms unit which was superb .