Could be because nobody knows the first thing about what kind of solder Husqvarna used on a Model 640 front sight ramp.
But it really doesn't matter.
If it's soft solder, and you heat it up to 450 degrees or so with a propane torch, it will fall off.
Silver solder melts closer to 1400 degrees, and you will ruin the bluing before you melt it off.
Course, you are going to ruin the bluing one way or the other if you take the ramp off, because there isn't going to be any under the ramp anyway.
rcmodel