A Watford speedometer measured in 'mins per 1000 ft' HELP!

I recently got hold of a Watford Speedometer but having searched the internet I have not been able to find any that are measured in the same unit which is 'mins per 1000 ft'
I bought the speedometer with a lot of projection equipment this makes me think that it should perhaps be for measuring film and not for a car or motorcycle but I have no clue

If anyone has any idea help would be much appreciated

 

Can you post the name of the company that looks to be under the No 115455.

 

Does the pully on the back drive it? If so, I must dismiss my first idea of a barometric rate of climb/descent indicator. If it is connected with the film industry, could it be that some film stock is rated higher than other film stock for rewind speed, and this gadget would be connected to the rewinding machine to ensure that the film was not snapped in rewinding by doing it too fast.

 

Then again, who speaks of stores but the military.

It may have been attached to a winch that deployed barrage balloons, although half an hour for a 1000 foot tether sounds a bit slow.

 

The company name is 'North & Sons Ltd'

 

How about a hydrographic application, measuring sea water temperatures at various depths, diffent retrieval speeds for the dangling themometer?

I can almost think of a thousand and one uses for such a gadget.

Just not cleaning carpets.

 

North & Sons Watford magnetos in graces guide for commercial vehicles and aeroplanes in 1933. LINK

 

looks like it was already asked and answered in 2014

 

Thousands of feet per minute is much more palatable

Minutes per thousand just seems so lethargic