Mohan Singh was first introduced to Rikhi Ram Sharma in 1940, when they both worked at Sher Mohammad Sitarmakers in Lahore. By 1943, Rikhi Ram's nephews, Kartar Chand Sharma and Hansraj Sharma, had joined Mohan Singh and Rikhi Ram at Sher Mohammad. This team built sitars together at Sher Mohammed until partition in 1947.
As a result of partition, Mohan Singh moved to Amritsar, and later to Jalandhar, Punjab, where he started the Mohan Singh & Company Musical Instrument Manufacturers & Suppliers, Phagwara Gate, Jullundur. (At about the same time, Rikhi Ram Sharma, Kartar Chand Sharma, and Hansraj Sharma moved to Delhi to start the Rikhi Ram Musical Instrument Mfg. Co. in Paharganj.)
In 1959, the same year that Rikhi Ram Sharma, Kartar Chand Sharma, and Hansraj Sharma broke up, Mohan Singh's nephew (but sometimes said to be his son), Gurdial Singh, took over Mohan Singh & Company, changing its name to Gurdial Singh & Sons.
The Registry's founder relies on an early 1950s Mohan Sign as one of his daily players. They can be spectacular instruments.
Sources: Most of the information in this post is from the Sitar Factory website, which you must experience if you love sitars. Please visit Klaas here.