Dear John,
Omshanti. These different forms of remembrance that were prevalent at various times from 1936 to 1969 were not the inventions of any BK, but was taught by Baba Himself depending on the grasping power of the children from time to time.
Before the beginning of the Yagya, the concept of God in the predominantly Hindu Indian subcontinent was either as a Shivling or in the form of various deities. So first of all, God made his mouthborn progeny realize that God was in the form of a Shivling and all other deities were souls and not Gods and that they were only deities before 84 births. Even in order to make the then mouthborn children realize the Godly task, He had to cause many divine visions to most of them.
When children understood the difference between God Shiv and the deities, then He explained them that God was not as large as a Shivling but was in the form of a thumb (angushthaakaar).
Later on when the Brahmin children became much more matured in the path of knowledge, He told them that God was not even in the shape of a thumb but only a point of light who, although a resident of the Soul World, comes down to enter and reside in the middle of the forehead of his human Chariot like all other souls. But He only takes loan of the human Chariot. Until Brahma Baba was alive, BKs used to remember Shiv through his body, but after his demise, the senior BKs started teaching the BKs to remember God Shiv as a point of light in the Soul World, since they believed that He had returned to the Soul World and there was no human Chariot left in this world. Even though they believe Shiv comes in the body of Gulzar Dadiji, they never remember Him through her body. Some BKs, due to their sanskars of past or present birth may prefer to remember Shiv as a point of light in the middle of the forehead of the non-living picture of Brahma Baba.
Since 1976 all those souls who started realizing that God Shiv was still on this Earth, but in another human Chariot (i.e. Shankar) they started remembering Him through the body of Shankar as per the directions given in the Murlis.
With regards,
On Godly service,
Arjun