I cannot tell you about that particular meter, but Weston is the most famous name in moving coil meters.
Edward Weston was an English born American who was one of the great geniuses of the late 19th and 20th centuries. He invented the way to make practical long-life carbon filament lamps, but due to the stupid patent laws, he had to give the technology to Edison who was famous for having the best lawyers.
Weston designed the first accurate moving coil meters, along with the mercury standard battery that was the standard for voltage for half the 20th century, zero temperature coefficient resistance wire and a heap of other things. He was a genius in Electronic measurement, Chemistry, Metalurgy - basically whatever he put his mind to.
Weston Electrical Instrument Co. of Surbiton, Surrey was the English subsidiary of the Weston Electrical Instrument Corporation in the US. In 1936 - the year of Edward Weston's death - it was acquired by British Sangamo. British Sangamo was a subsidiary of the Sangamo Electric Co. of Springfield, Illinois.
Sangamo Weston was sold to Schlumberger in 1976.
This meter was built in an era when people worked at the one company for their working life, so for sure, the designers of this meter had worked directly with Weston's designs, and perhaps worked directly with Weston. Looks like a very fine meter, and I am sure it is still a great meter to use.
I would guess the 1.5V battery is a D cell. Not sure about the 4.5V battery. The common one for the 60's was the 4.5V Bicycle Lamp battery that looks like it is still available:
http://www.master-instruments.com.au/products/61491/MN1203.htmlNot sure how it would connect to the terminal posts unless the battery fitted into a removable holder that is missing now.
Richard.