I suspect that you will need a lot more detail to get a good answer. Some time ago, TradeEx sold M5 x 0.8 plugs - for some Euro side mounts. No clue if that is what your holes are - could be M6, could be 1/4". Then multiple counts of threads per inch available, for each diameter size. Then finding a Euro style side mount with that exact hole spacing for the base. Knowing exact size of screws and their thread designation, and exact center-to-center spacing of those holes is pretty important, I think, if you want to find a mount that fits to that. Is a good chance if those holes were drilled and tapped in Sweden, they are likely a metric size - if done by someone here in North America - could be whatever the person had handy?? Was also a thing for some years to install BA threaded screws for scope bases, so is that possibility as well.