Stop locks were created where the canals of two different companys met. each company was protective of "their" water and wished to ensure tolls were paid by boats entering or leaving. Often, there was very little change of depth; in the case of Hall green it is only 4 inches.
Clare Cuisers' boatyard can be seen at the left of the picture.
This is the only boatlift in Britain. Lifts or inclines were used where a large change in height was needed over a short horizontal distance. Since the photo was taken, the lift has been renovated and returned to its original 'hydraulic' operating method