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One of my most studied models of all the 1-75 miniatures and I am pleased to report some new and unrecorded details.
I do not have an engineer’s 21d model or a 21d pre-pro but after studying dozens and dozens of photos I have found some proof that the designers/toolmakers had more ideas that were not used in production. I have also written to a collector who has a model in hope that we can see a clearer photo.

This time it relates to the early stages of development where I believe the mounting or securing of the barrel axle was to be secured by a combination of a chassis (base) mounted support along with the cab support.

I had long wondered why the cab had a cut-out at the rear and a smaller second cut-out. I knew it had something to do with the pin axle but to come into play the axle had to be longer.

And the answer is......
On the pre-production castings there is a perpendicular support on the chassis ( base ) that also has a tiny semi circle cut-out and when the chassis and the cab come together this tiny opening allows for a longer axle to fit through.
If you look closely at image 4 you can see the up-stand on this is an engineer's model that we saw many years ago on the forum and this was an ebay sale photo I recovered. It is not very clear but was enough to research further.

I think R&D realised very quickly that this was not necessary and the chassis support was removed from the chassis mould.

In fact they also soon discovered there was no need for a steel axle at all and the barrel could still function with a plastic pin under the hopper arch.

However I have yet to find a broken 21d that has the steel pin whereas the many broken ones can be found with the plastic pin.

Bert

Absolutely fascinating, Bert, and great detective work.
I must admit to always having found the concept of R&D within Lesney to be a strange idea. After all, how can you not design a 1-75 correctly? How difficult can it be? Surely it would be first time right, every time? However, as we see time and time again on this Forum, modifications were required in order to strengthen and improve models, and eliminate design flaws. What your post above shows is that there was a significant design and development effort long before a model made it to the shops. Despite over a decade of experience, mistakes were still being made and design dead-ends were being included and then removed during a model's development phase.
If only we could have more stories like this one!

I wonder how much turnover was in that department. Employee turnover can often render past lessons learned into lessons lost. But, sometimes they bring in new ideas not previously considered.

Tinman wrote:I wonder how much turnover was in that department. Employee turnover can often render past lessons learned into lessons lost. But, sometimes they bring in new ideas not previously considered.

That's a good point.
I work in the chemical processing industry, and corporate memory is supposed to be something like ten years (which is why accidents with the same root cause occur again and again in the same organisation).There is ample evidence that Lesney made the same design mistakes over and over. (Think of the addition of strengthening to the underside of the front bumper below the baseplate tab slot on the 46a and the later on the 10c, and the multiple cases of triangular bracing webs being added to rivet posts.)
Whilst Jack Odell was very obviously a highly-skilled craftsman with the ability to cleverly pack a lot of detail into a limited space in order to create masterpieces in miniature, I think his grasp of stresses and strains was probably extremely limited, which is why we regularly see strengthening being subsequently added to designs. Those design flaws are something R&D should have picked up on during model design and development, but didn't, supporting the idea that departmental staff turnover was high enough to prevent meaningful corporate learning.

Some very interesting information Bert, I have a few of these but none with that casting. I will keep an eye out as always.
Jason