Om Shanti. In order to understand the history related to the importance of the year 1976 from a BK/PBK perspective, it is important to read the declaration made in the picture of Lakshmi-Narayan published by the BKs during the lifetime of Brahma Baba, but after Om Radhey Mama had left her body.

- Lakshmi-Narayan picture
At the bottom of the picture (published by BKs in English) just below the image of Radha and Krishna it is written:
"Godly Message -
Corruption, Irreligiousness, Unrighteousness, vices, insolvency and sufferings would come to an end in Bharat within 9 years and Golden Aged Deity-World-Sovereignty of Shri Lakshmi and Shri Narayan will come to be re-established soon after the forth-coming huge world destruction. June 1967.
Prajapita Brahmakumaris and Brahmakumars"
The above picture was in most probability prepared on the basis of visions caused to trance messengers (BK sisters going into trance called sandeshis) and hence were believed by BKs and Brahma Baba to be declarations made by ShivBaba. Based on the above declaration, Brahma Baba and BKs started believing that the world would come to an end in 1976. And with every approaching year, hints were given in the Sakar Murlis and Avyakt Vanis that we are such and such number of years closer to destruction. BKs started declaring and propagating in the world that the world is going to be destroyed in 1976. Many BKs might have sold their properties and surrendered to the Yagya. BKWSU also most probably gave in writing to the Government of India that the world is going to be destroyed in 1976. But when nothing of that sort happened in 1976, many BKs were devastated. Many left the knowledge. But many continued.
Until 1976 BKs had not acquired properties anywhere outside Mount Abu. They functioned only from rented buildings. But after 1976 they started acquiring properties in the name of BKWSU or World Renewal Trust. And they stopped giving any dates for destruction although they did and still propagate that the world is going to be destroyed.
From the PBK point of view, the above declaration was not made in a physical sense but in a metaphorical sense. The destruction referred to in the declaration was actually the beginning of destruction of wicked elements from the Yagya. The Lakshmi-Narayan referred to in the picture are the Confluence-Aged Lakshmi and Narayan whose parts were revealed to many in the Brahmin world. Destruction of vices and irreligiousness referred to in the above declaration was actually the beginning of the destruction of vices in the minds of the souls which are going to be revealed in the world as Lakshmi-Narayan or Bharat Mata and Bharat Pita (living Bhaarat).
It is also pertinent to note that the year 1976-77 was also declared as the year of revelation of the Father in many Avyakt Vanis narrated before that year. The BKs were unable to reveal the Father (BapDada) to the world in that year, but some Brahmin souls realized the practical part of Father Shiv being played through Baba Virendra Dev Dixit in 1976-77. Therefore the declaration related to 1976 is very important in the history of the Yagya (of BKs as well as PBKs).
If other PBKs have time they can post the relevant Murli or Avyakt Vani points. It may take time for me to post the points.
OGS,
Arjun