The construction design of all Boehm Models is basically the same, and it seems also not to have changed during the years: From 1905 on (when the standard models were introduced), the basic measurements and the bracing on the inside have always been the same (small variations in size seem to be rather due to the production by hand as due to a conscious will to variation). But it looks as if Boehm increased the thickness of the (spruce) tops of his instruments when some older instruments had begun to warp or to crack under the tension of the strings. Such an increasing of the thickness of the top is at least suggested by the measurements which the Hamburg guitar maker Marius Redeker made for his bachelor thesis about the Boehm Waldzither.
Some more information about the construction measurements of the Boehm instruments can be found in the catalogue: "Historische Musikinstrumente der Staatlichen Reka-Sammlung im Bezirksmuseum Viadrina Frankfurt/Oder" (Leipzig 1989) by H. Heyde:
Because of their identical construction, many Boehm instruments also show similar signs of age or storage. Especially Waldzithers that were unprofessionally stored (i.e. too dry, too damp, with changing temperature or tensed strings) often display tension or shrinkage splits, their neck is warped or the top has sunken. In other cases, the strings draw the tailpiece into the body or the struts push the sides of the instrument outward.
The glass bridges also have their share in producing cracks, for they are almost never exactly adjusted to the (slightly arched) top. In many cases, the bridges are supported at both ends with small pieces of cardboard or wood (it almost seems as if Boehm himself did this at times), with the effect that the string pressure is not evenly transferred to the top (what of course produces tensions).
Another striking fact is that the top Model Nr. 4 (of all things!) was structurally weakened by Boehm himself: On these instruments, the inlays at the edge of the top and the back are much wider (which means part of the top wood had to be cut off), but Boehm nevertheless used exactly the same bracing as on the other models – to the effect that the top is no longer supported by the linings. Because of this weakening of the structure, the top has broken loose from the side of the instrument in quite some cases.