From the picture on the LoC site, the bar layout is that of a strohfidel rather than a modern xylophone - the bars are aligned left to right in overlapping rows, like the strings of a hammered dulcimer. As its name suggests, a strohfidel has the bars supported on bundles of straw lined up crosswise along the nodal points.. Gutta percha would be less bulky. I'd guess Haselmayer's only innovation was to use that.