I've had another think about this, and I've realised why it needs to be a physical stop that interacts with the finger of the dialler; it's because it has to work the same way, irrespective of which number you select. A stop in the dial mechanism could provide a limit to the maximum rotation that the dial could perform, but it would not be able to ensure that for each number dialled, the dial was moved the correct amount and the correct number of impulses were generated.
The impulse telephone is basically like a wind-up monkey, and every time the cymbals clash, a connection is made and an impulse sent down the line. There may be some kind of impulse at the start of each number to tell the system to start listening for a number, but essentially, for 1, you need 1 pulse, for 5, you need 5, and for 0 you need 10 pulses to be sent down the line. So you need each number to 'wind the monkey' the correct amount and upon release the little cymbals crash, the requisite number of times. If you were to put your finger in the '9' hole in the dial, but instead of rotating the dial to the stop, you just rotate it to the position of the '5', then you wouldn't get the correct number of pulses - you'd most likely only get 4 or 5. A mechanical stop wouldn't be able to do this - since irrespective of which number you selected, the mechanical stop would stop the dial after the same amount of rotation - presumably you would dial '0' regardless which number you chose. I suppose you could have sprung number buttons in the dial face, which protrude downwards when selected by the operator and then interact with a hidden stop hook when you rotate the dial - but all this is doing is adding a layer of complication between the user's finger and the stop hook, increasing the chance of something breaking whilst delivering unclear benefits. You could also have a coloured 'end zone' to indicate to the user that they have reached the end of the rotation and should release the dial, but I think this would be more prone to user error as people dial absent-mindedly.