Pluck's frame study indicates that Ford originally stamped in the stops. After a time they stopped stamping them due to the change of the E-brake lever and started bolting them but that was also problematic so they started stamping them again. From what I could find, there was likely a period where there were no stops but this led to problems with the swinging equalizer beam so they started bolting the stops but that was likely a stop gap measure till they could start stamping them again.
This system was problematic for Ford and was not legal in the state of Pennsylvania. Ford finally developed the fully independent emergency brake system in late 1928 to allow sales in all 48 states.