I think that this will be very rare, and made well before 1960.
I worked for James A Sinclair in the early/mid 60's, they had a very olde worlde shop at No. 3 Whitehall, complete with massive vaults below, from the shop's previous life as a bank. They also had a small factory (more of a workshop really) in Highgate, where all of their own make cameras were made.
The retail shop basically served the aristocracy and the senior civil servants who worked nearby, but the main business was their 35mm cameras. These were entirely hand made, and the telltale diamond shaped etching on the alluminium cases was rubbed with a cork dipped in carbide paste, running along a straight edge.
Many years before I joined them they made various models for the movie industry, and the one mentioned here would be one of these - they had a shelf full of them, on display at the factory, although I wasn't allowed to touch them, I do remember asking questions about them and having a good look.
But by the time I joined them, their main customer was HMG, with cameras supplied to the RN, RAF and the army. The special models made for the Royal Navy were the most interesting, as they had a special and unique feature (still covered by the Official Secrets Act) that made them perfect for use by navy divers.
The models that were current when I worked for them were available with 2, 3, 4 or 5 turrets, and most of their lenses were by Angenieux, probably because Sinclair's had a relationship with Pierre Angenieux. Some didn't have a turret at all, and were fitted with what was then a revolutionary Angenieux zoom lens, which from memory had a range of 10:1.
I still have fond memories of Sinclair's, and the staff who worked there. I learned a lot from them and then went on to spend my working life as a photographer. I was very young at the time I was there, and did most of the running around jobs, which took me into the world of their wealthy private customers (I was assistant when we shot movies of society weddings for customers) and also inside various defence establishments, mainly the Admiralty.
So, I can't help much with details about this particular camera, but I hope that my experience is helpful.
