That is an odd hole. It doesn't look like it is a rust pit. It looks like it has been gouged with something. Screw?

Is it just on one fork blade, or on both?

What are your ultimate plans for the bike? A few Sunday rides? Hardcore commuting? Resale?

The photos look like it is solid metal around the hole. If it was me, I'd probably braze or tig it, and many of the old metals are supposed to do better with braze temperatures than tig temperatures, so it would probably get a drop of brass or silver.