As you will see from the 'derailleurs' tab, this collection includes a derailleur that is branded on its outer parallelogram plate as 'Diamond'. Its inner parallelogram plate carries the words 'Shanghai China'. Finally it features a fine logo of a glittering cut diamond with rays of light emanating from it.
It's hard to find any information about this, but a Chinese correspondent recognised the logo as being very similar to that used on the popular 'Diamond' range of watches which were also made in Shanghai. Apparently Diamond watches were famous in China for their quality. He speculated that the derailleur was manufactured by the Shanghai ZuanShi Watch Factory - the proud manufacturer of Diamond brand watches. If I understood his email correctly his father had worked for the Shanghai ZuanShi Watch Factory. You have to be a touch careful about this - a logo of a glittering diamond with rays of light emanating from it is not that uncommon - but let's run with it.
I believe that the Shanghai ZuanShi Watch Factory was founded in 1932 as the Tak On Clock Factory, then became the Jin Xing (Gold Star) Industrial Company. Following the revolution, it was nationalised to become part of the Shanghai Clock and Watch Industry Company. Finally, in 1986, with the rise of Deng Xiaoping's 'Socialism with Chinese characteristics', it was renamed Shanghai ZuanShi Watch Factory. At this point it seems that the watch and clock market was liberalised, and it is possible that the company was privatised. The Shanghai ZuanShi Watch Factory struggled to compete in this new environment and appears to have been liquidated in the mid 1990s. A company making stopwatches, called the Shanghai Diamond Stopwatch Company, which shares much of the same lineage, survived and is still active today. It continues to use the 'Diamond' brand and logo.
My correspondent suggested that the Shanghai ZuanShi Watch Factory may have produced some bicycle parts in its final years (possibly around 1993-1995?). At this point the company was struggling, and the booming Chinese bicycle industry offered a tempting new market that might appreciate its precision engineering skills. The Shanghai area was, and is, an important centre of bicycle manufacture.
This is all highly speculative, and I have no concrete evidence for any of it, but it does seem plausible or, at least, possible.