The Commonwealth was a US design, from Commonwealth Steel of St Louis. Behind the scenes they had long been jointly owned by the largest two USA steam loco builders, Baldwin and Alco. Their specialism was substantial steel castings, likely larger than any in-house foundry at a railway workshop could manage, hence the large heavyweight frame of the Commonwealth bogie, rather than a built-up/fabricated one. They were also experts in developing the frame casting into a full bogie design, with all the springing, dynamics etc understood. Into the diesel era General Motors and others made extensive use of their bogies, typically because they had been specified by the purchaser.
In Britain, they licensed the design patents not to BR but to a steel company in Sheffield who had the casting capability. The bogie was expensive because there were thus several companies in the chain, and also because it was physically heavy with a lot of solid steel in it. Do I believe they were about 2 tons each heavier than the B4. As described above, it rode better than anything else around, given some expert maintenance, which part of the high price made sure was well documented, right tools, etc. It's the old story. You can have a big, heavy, expensive product that works well, or a supposedly more efficient, lighter, cheaper, easier to make one that doesn't quite get there. Designers trying to get the upsides of both approaches in one product rarely achieve much more than a compromise.
I think pretty much all the main line Mk 1 stock built by BR in 1960-63, between the B1 falling out of favour and the B4 coming along with Mk 2 stock, used the Commonwealth. When the stock was withdrawn the bogies seem principally to have then been used again secondhand as replacements on other vehicles.
