Disclaimer: I'm no professional piano tech, but I can share what info I have.
To my knowledge, Dietmann was a German piano technician who emigrated to South Africa in the 1920s. His son founded the piano factory in Wellington around 1955. They built pianos at various price and quality points, some of them stencils, some under license (e.g. Ibach - I have one of those). The factory closed in the 80s.
As far as I've been able to determine, most of the factory's brands, Dietmann amongst them, had a good reputation as reliable instruments, and were actually exported worldwide. But it appears they used a wide range of parts and qualities.
* The Ibachs, for example, were their flagship, and were assembled with Renner actions and Ibach soundboards (at least they bear the genuine decal) from Schwelm in Germany. To my knowledge, Rolf Ibach actually worked with (or at?) at the factory for some time. My parents also have one of these (1966), similar to mine (1970). While both sound very good and play well, they show signs of shoddy workmanship, notably sloppy bridge notching and pinning.
* The Otto Bachs were apparently their mid-range and "work horse". Every other household and teacher had one, and still has. I was told by an elderly tech that these were actually stencilled Knight pianos. "Otto Bach" was originally an export range of the big Leipzig manufacturer Gebr. Zimmermann. (I have one of those "originals".) Apparently Dietmann jr. bought the name and stencilled it onto those Knights. They were sold by the thousands and are still very common on the local used piano market. I've worked on one, and saw that it had the stamp of an English keyboard maker on the keyframe - but I've forgotten the name. This would bear out the assertion that they are stenciled English pianos.
* Another brand that I've seen from this factory is Ludwig Meister, but I don't know anything about these. They may have been an entry-level offering.
* Whether the Dietmann range was some stencil or Dietmann's own design and product, I don't know. They are not as common as the Otto Bachs.
The Otto Bach that I worked on also had some exploded hammers.
I glued and clamped this with hot hide glue, but didn't fit a (new) staple (yet). I see the piano regularly, and as yet, the repair is holding up. The action has no markings, so I presume it was developed in-house by the Dietmann factory. It has black plastic flanges and jacks, which are holding up well even after 40 years. (The keyframe punchings are another matter...)
The other weak spot on that instrument is a severely cracked bridge in the high treble. This may be an isolated case - I have no others to compare it with. It's still tunable, but I'm envisaging to repair it with epoxy sometime. The pinblock tunes well, but may be separating in the high treble, as the right action bracket is jamming quite severely on its bolt. The soundboard was varnished/painted with a greenish, almost opaque finish, and is crack-free.
I'm just providing these details so that the OP can compare notes.
I still have the original Dietmann factory brochure and invoice that came with my Ibach (the previous owner bought it new). I'll have a look whether they wrote something specific about the various brands they sold.
Sorry, most of this is second-hand information that I've gleaned from local techs, internet searching and partly my own observations. Some local techs may know better, but I haven't seen any such info posted on the 'net, which is why I've summarised my own findings here. I hope it helps the OP nevertheless.