Googled "wiping solder" and there were several hits. It seems there were several different ratios available. This doesn't help answer you original question but you might find something if you keep digging.

If you have other bars of certified solder you could do test impressions against this to check relative hardness. To do this use the ball bearing between the two pieces known and unknown and squeeze in a vice and compare impression depths. Again this still only gives you a relative idea of hardness not elemental make up. Just an idea.

I've always believed that wiping solder was 60/40.  According to a poster on Cast Boolits, during WW2 the tin shortage was so bad that the substituted silver.   That's all I can remember.

All the "body solder" or wiping solder I have is 30-70 and so marked on the bars.  Before Bondo that's what you used to fill in seams and dents you couldn't hammer out.  30-70 has a wide slushy range and you pushed it around with maple paddles, adding a little heat from a torch as necessary.

Instrumental name:

Energy-dispersive X-ray spectroscopy sometimes called energy dispersive X-ray analysis (EDXA).

The other candidate is: X-ray fluorescence. 

You might try melting point but the actual melting point may be rather subjective.

Just using those bars as a source  of Sn might be the best approach. Formulate a small batch until you get the hardness you want then commit the rest to a final batch. 

as with everything like this YMMV.