They are just blind plugs that connect to the oil gallery a few mm away that you already have access to thru the min hole. These plugs are cast in for NA check valves.
The key to cleaning the block is to riffle brush as many holes as you can, let your machine shop hot tank it with the freeze plugs out and use their high pressure steam bath and water jet block cleaner. IMO if your chosen machine shop does not have this, find one that does. Just before assembly I wash out all all galleries with brake clean and blow it clean with compressed air. I also use shellac for freeze plugs on the inside as a precaution and to prevent corrosion with oem plugs.
