Early Minis had the take-off at the rear of the block above cylinder 4. The heater circuit ( if the car had a heater) returned to the lower radiator hose. (Cars without heaters would have used the lower hose without the side branch.) The by-pass hose between the block and the engine was to permit circulation of coolant across through the head passages until the thermostat would open, to avoid hot spots and possible head gasket issues. In other words, the by-pass hose by-passed the thermostat. Not much of an issue unless the heater valve was closed, since the heater circuit also by-passes the thermostat.

Later engines omitted the by-pass hose, probably because variations in heater hose arrangements, sandwich plates etc. eliminated the risk of hot-spots. For engines without the by-pass or the sandwich plate, the general recommendation is to drill a few small holes in the thermostat to act as a by-pass. However some think it not necessary, since modern thermostats are "fail-safe" - when they fail, they fail in the open position, alleviating (not eliminating) the risk of head overheating. If the head gets hot enough to boil coolant, the thermostat will open before it actually does. I imagine the newer head passages may also have been modified to suit this.
 

Early minis have a heater hose outlet port on the rear of the head, and a "by pass" hose between the head and the block. The by pass hose is used to allow coolant to circulate, before the thermostat opens.  Later minis use a sandwich plate, no "by pass" hose and a different type lower rad. hose. The later lower rad. hose has a provision  for a heater hose outlet, and a bypass connection. My question is WHY did they make this modification, and would this modification be of any benefit to earlier minis.  Thanks in advance Jay