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This instrument was taken apart entirely, thought out anew and assembled again differently; a kind of a musical laboratory.
The Weltklang baritone 3536 was bought with the clear intention to use it as a starting point for a far-reaching reconstruction. It turned out to be a project that does make it clear what can be gained when one starts from a point where there's nothing left to lose.
A weak point of these instruments certainly is their intonation. The bore profile is chaotic. We find unfavourable joints between pieces of tubing and a lot of relative widenings and narrowings follow one another. These faults cause serious intonation problems. Along with that the instrument sounds dull and stuffy because of a rather low key height and a narrow neck entrance. Changes were made to both bore profile and mechanism. Only very few things remained original.
There we go... An overview of the foremost changes made, from top to bottom: 
In all, three necks were built for this instrument, which have, all of them for as long as they are, about the same bore profile. The lengths of these necks are different though. Also, the longest of them carries a third register key. Thus the baritone acquires a bassoon–like range.
A separate page is devoted to these three necks; see a study in truncation.
Extensive experiments were conducted with the bore profile of the upper bow, which is of utmost importance to the correct octaves of notes of the right hand. Here the upper bow was cut open lengthwise and trimmed to size with wire and tape.
Once the correct profile was established, the bow was made to size.
An overview of the top end of the instrument seen from behind. This is the longest neck with a third register key. The picture also shows that the upper bow and the crook got a different position relative to the body of the instrument so as to make the point were the neck springs from is on the right 'outside' of the instrument instead of behind it.
The top end of the instrument was fortified with a thick glue lined heat shrink tube. This makes the wall of the instrument acoustically inert and thus enhances a quick response (especially in the low range) and a more even sonority.
A separate page is devoted to this curious phenomenon. See under the heading inertia
The register mechanism was made entirely anew. Mark the ball joints in the swivel, which were made adjustable with setscrews. The arm in the upper left corner operates the third register key.
The bridge between keys 10, 11 (G#) and 15 (Bb) was unlinked from the key cup itself and equipped with two setscrews.
The bell tones were annoyingly sharp. The rigorous solution proved to be the best one: the bottom bow was lengthened with twice 16 millimeters and fitted back again. Here too the bore profile had to be modified.
The bottow bow after its lengthening. Mark that a mount point for a pin has been added too.
The spacious triangular bell brace consists out of two sheets of brass with a thickness of 1½ millimeter. The text reads as follows:
this is not a WELTKLANG
2004 : instrument modified : 2008
M. Postma Yzendijke
intonation, as it was and after all the changes made.