definaltely looks like a bar of solder. My guess is that its probably 60/40 but it could be about any combination of lead and solder. Bend it and if it brakes clean without bending it is probably tin but if it bends some before breaking its probably a tin lead mix. Another way to test it is to melt it and watch temps but thats pretty tough. A guy could also get some known pure tin and pour some in a bullet mold and do the same with your bar and compare weithts of the two.
