It proves difficult to obtain correct information about the developments of the vanguard cylinder head. I found a number of details about the development of the TR heads.(For example the Terry-Ann homepage gives a good overview at http://www.expeditionlandrover.info/2trSite/FAQ_heads.htm) Whether the reasons for design changes given at some internet references are always correct is doubtful, because from a flow point of view the TR heads with their squaire outlet ports really are not better than the vanguard heads having round ports. Port sizes are similar on the different models. Only the later so called "high port heads" has some advantageous flow features.

I have two vanguard heads which are quite different. The apparently later one has a higher compression, (head is more than 3mm less high than the earlier one) larger inlet ports and a diagonal slant on the underside of the lower thermostat housing like the late TR3 models.
Who knows more??