holes on the inside of metal fork blades, what are they
On steel forks the tiny holes at the bottom end of the fork-blade are vent holes for the brazing process. The brazing produces fumes the fumes and the heat expands the air in the blade which makes that these holes are needed to evacuate both. On steel frames you may find similar holes in the seat-stays and in the chain-stays where they are usually close to rear drop-outs.
If you're worried about rusting because of moisture that could go inside spray some wax-coating or tar-based antirust inside, or a periodic jet of penetrating oil as I do.