[morzh]

It is weird it did not melt off. Yes I've heard (and seen) wire welding, but that was in Soviet radios, and I've never seen welding in the civilian radios in the US, let alone to the coil solder tab.
I still think it is solder, but it could be oxidized, and I often see that oxidized solder is not easily melted by even a very hot soldering gun.
If I need to melt that kind of solder, I apply molten solder with flux (rosin core solder wire) right to the spot with hot enough solder tip.

This said, I rarely (if ever) try to desolder wires at the solder tabs on coils: the coils are easily damaged.

In your situation, what I would do:

- take a scalpel, scrape the solder tab (solder or the metal where there is no solder, but which would be accessible to the solder tip) and see what color it is. You should be able to see if it is copper/phosphor bronze or something that is yellow/reddish, or it stays greyish white, which would indicate solder.
If it is metal contact, scrape it well down to bare metal, and solder to it with usual tin-led solder. If it is solder, scrape well until shiny, and try to solder with tin-let-rosin core, at somewhat higher (not too high) temperature, making a bead; then solder wire to it.

I do not claim to have seen everything; there are folks here who have seen tons more than I have; so maybe they will have more ideas about what your contact is, though this being a 60 (have just finished one and worked on another one before, with coild rewinding), I doubt it is anything other than regular contact with regular solder.